clang: Define / releative gcc installation dir

This is required for OE gcc installation to work.
Without this its not able to find the paths for libgcc
and other standard headers and libraries from gcc
installation in OE

* Do not use install relative libc++ headers
In OE we use same clang for native and cross builds, therefore we need
to ensure that native sysroot install of libc++ is not searched for
headers when doing cross compile instead it searches the target sysroot
this is especially troublesome when libcxx-native is staged along with
libcxx e.g. chromium

 * Fix lib paths for OpenEmbedded Host

Under OpenEmbedded Host, while building with clang-native, it cannot find
the GCCInstallPath, which causing following error:
[snip]
compiler-rt-native/13.0.1-r0/recipe-sysroot-native/usr/bin/clang
-target x86_64-linux
-isystem/path/to/x86_64-linux/compiler-rt-native/13.0.1-r0/recipe-sysroot-native/usr/include
-O2 -pipe
/path/to/compiler-rt-native/13.0.1-r0/recipe-sysroot-native/usr/share/cmake-3.21/Modules/CMakeCCompilerABI.c`
hosttools/ld: cannot find crtbeginS.o: No such file or directory
[snip]

Before this patch:
compiler-rt-native/13.0.1-r0/recipe-sysroot-native/usr/bin/clang
clang version 13.0.1 (https://github.com/llvm/llvm-project 08e3a5c)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /build/tmp-glibc/work/x86_64-linux/compiler-rt-native/13.0.1-r0/recipe-sysroot-native/usr/bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-wrs-linux/10.2.0

After this patch:
compiler-rt-native/13.0.1-r0/recipe-sysroot-native/usr/bin/clang
clang version 13.0.1 (https://github.com/llvm/llvm-project 08e3a5c)
Thread model: posix
InstalledDir: /build/tmp-glibc/work/x86_64-linux/compiler-rt-native/13.0.1-r0/recipe-sysroot-native/usr/bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-wrs-linux/10.2.0
Found candidate GCC installation: /usr/lib/x86_64-wrs-linux/10.2.0
Selected GCC installation: /usr/lib/x86_64-wrs-linux/10.2.0
Candidate multilib: .;@m64
Selected multilib: .;@m64

For OpenEmbedded Host, sysroots are of the form<sysroot>/usr/lib/<triple>/x.y.z.
Take x86-64 as example, the default triple is x86_64-unknown-linux-gnu.
For clang-native, the target vendor is '-unknown', need to test current distro
to follow above form.

Upstream-Status: Inappropriate [oe specific]

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>

Author: kraj

clang/lib/Driver/ToolChains/Gnu.cpp

@@ -19,6 +19,7 @@
 #include "Linux.h"
 #include "clang/Config/config.h" // for GCC_INSTALL_PREFIX
 #include "clang/Driver/Compilation.h"
+#include "clang/Driver/Distro.h"
 #include "clang/Driver/Driver.h"
 #include "clang/Driver/DriverDiagnostic.h"
 #include "clang/Driver/MultilibBuilder.h"
@@ -2860,6 +2861,7 @@ void Generic_GCC::GCCInstallationDetector::ScanLibDirForGCCTriple(
     const llvm::Triple &TargetTriple, const ArgList &Args,
     const std::string &LibDir, StringRef CandidateTriple,
     bool NeedsBiarchSuffix, bool GCCDirExists, bool GCCCrossDirExists) {
+  Distro Distro(D.getVFS(), TargetTriple);
   // Locations relative to the system lib directory where GCC's triple-specific
   // directories might reside.
   struct GCCLibSuffix {
@@ -2871,19 +2873,20 @@ void Generic_GCC::GCCInstallationDetector::ScanLibDirForGCCTriple(
     // Whether this library suffix is relevant for the triple.
     bool Active;
   } Suffixes[] = {
-      // This is the normal place.
-      {"gcc/" + CandidateTriple.str(), "../..", GCCDirExists},
-
-      // Debian puts cross-compilers in gcc-cross.
-      {"gcc-cross/" + CandidateTriple.str(), "../..", GCCCrossDirExists},
-
       // The Freescale PPC SDK has the gcc libraries in
       // <sysroot>/usr/lib/<triple>/x.y.z so have a look there as well. Only do
       // this on Freescale triples, though, since some systems put a *lot* of
       // files in that location, not just GCC installation data.
       {CandidateTriple.str(), "..",
        TargetTriple.getVendor() == llvm::Triple::Freescale ||
-           TargetTriple.getVendor() == llvm::Triple::OpenEmbedded}};
+           TargetTriple.getVendor() == llvm::Triple::OpenEmbedded ||
+           Distro.IsOpenEmbedded()},
+
+      // This is the normal place.
+      {"gcc/" + CandidateTriple.str(), "../..", GCCDirExists},
+
+      // Debian puts cross-compilers in gcc-cross.
+      {"gcc-cross/" + CandidateTriple.str(), "../..", GCCCrossDirExists}};
 
   for (auto &Suffix : Suffixes) {
     if (!Suffix.Active)
@@ -3274,8 +3277,11 @@ Generic_GCC::addLibCxxIncludePaths(const llvm::opt::ArgList &DriverArgs,
   // incompatible with the NDK libraries.
   SmallString<128> DriverIncludeDir(getDriver().Dir);
   llvm::sys::path::append(DriverIncludeDir, "..", "include");
+
+  // do not add it when --sysroot is specified, since it would expect
+  // libc++ headers from sysroot and not relative to compiler install location
   if (AddIncludePath(DriverIncludeDir,
-                     /*TargetDirRequired=*/getTriple().isAndroid()))
+                     /*TargetDirRequired=*/getTriple().isAndroid() | !computeSysRoot().empty()))
     return;
   // If this is a development, non-installed, clang, libcxx will
   // not be found at ../include/c++ but it likely to be found at