[ARM] Improve arm_neon.h header diagnostic when included on unsupported targets (#147817)

The footgun here was that the preprocessor diagnostic that looks for
__ARM_FP would fire when included on targets like x86_64, but the
suggestion it gives in that case is totally bogus. Avoid giving bad
advice, by first checking whether we're being built for an appropriate
target, and only then do the soft-fp check.

rdar://155449666

Author: jroelofs

clang/utils/TableGen/NeonEmitter.cpp

@@ -2417,7 +2417,11 @@ void NeonEmitter::run(raw_ostream &OS) {
   OS << "#ifndef __ARM_NEON_H\n";
   OS << "#define __ARM_NEON_H\n\n";
 
-  OS << "#ifndef __ARM_FP\n";
+  OS << "#if !defined(__arm__) && !defined(__aarch64__) && "
+        "!defined(__arm64ec__)\n";
+  OS << "#error \"<arm_neon.h> is intended only for ARM and AArch64 "
+        "targets\"\n";
+  OS << "#elif !defined(__ARM_FP)\n";
   OS << "#error \"NEON intrinsics not available with the soft-float ABI. "
         "Please use -mfloat-abi=softfp or -mfloat-abi=hard\"\n";
   OS << "#else\n\n";