[lldb] [CMake] Disable GCC's -Wstringop-truncation warning. NFC.

This warning gives false positives about lldb's correct use of
strncpy to fill fixed length fields that don't need null termination,
in lldb/source/Plugins/ObjectFile/Mach-O/ObjectFileMachO.cpp, like this:

    In file included from /usr/include/string.h:495,
                     from /usr/include/c++/9/cstring:42,
                     from ../include/llvm/ADT/StringRef.h:19,
                     from ../tools/lldb/source/Plugins/ObjectFile/Mach-O/ObjectFileMachO.cpp:10:
    In function ‘char* strncpy(char*, const char*, size_t)’,
        inlined from ‘lldb::offset_t CreateAllImageInfosPayload(const ProcessSP&, lldb::offset_t, lldb_private::StreamString&, lldb::SaveCoreStyle)’ at ../tools/lldb/source/Plugins/ObjectFile/Mach-O/ObjectFileMachO.cpp:6341:16:
    /usr/include/x86_64-linux-gnu/bits/string_fortified.h:106:34: warning: ‘char* __builtin_strncpy(char*, const char*, long unsigned int)’ specified bound 16 equals destination size [-Wstringop-truncation]
      106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
          |          ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~

The warning could be squelched locally with

    #pragma GCC diagnostic ignored "-Wstringop-truncation"

too, but Clang also interprets those GCC pragmas, and produces
a -Wunknown-warning-option warning instead. That could be remedied
by wrapping the pragma in an "#ifndef __clang__" - but that makes
things even more messy. Instead, just silence this warning entirely.

Differential Revision: https://reviews.llvm.org/D123254

Author: mstorsjo

lldb/cmake/modules/LLDBConfig.cmake

@@ -183,6 +183,9 @@ append_if(CXX_SUPPORTS_UNKNOWN_PRAGMAS "-Wno-unknown-pragmas" CMAKE_CXX_FLAGS)
 check_cxx_compiler_flag("-Wstrict-aliasing" CXX_SUPPORTS_STRICT_ALIASING)
 append_if(CXX_SUPPORTS_STRICT_ALIASING "-Wno-strict-aliasing" CMAKE_CXX_FLAGS)
 
+check_cxx_compiler_flag("-Wstringop-truncation" CXX_SUPPORTS_STRINGOP_TRUNCATION)
+append_if(CXX_SUPPORTS_STRINGOP_TRUNCATION "-Wno-stringop-truncation" CMAKE_CXX_FLAGS)
+
 # Disable Clang warnings
 check_cxx_compiler_flag("-Wdeprecated-register" CXX_SUPPORTS_DEPRECATED_REGISTER)
 append_if(CXX_SUPPORTS_DEPRECATED_REGISTER "-Wno-deprecated-register" CMAKE_CXX_FLAGS)