[ARM] Improve arm_neon.h header diagnostic when included on unsupported targets

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 +2417,9 @@ 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";