[ARM] Improve diagnostics message when Neon is unsupported

[vhscampos]

Summary

Whenever Neon is not supported, a generic message is printed:

error: "NEON support not enabled"

Followed by a series of other error messages that are not useful once
the first one is printed.

This patch gives a more precise message in the case where Neon is
unsupported because an invalid float ABI was specified: the soft float
ABI.

error: "NEON intrinsics not available with the soft-float ABI. Please
use -mfloat-abi=softfp or -mfloat-abi=hard"

This message is the same one that GCC gives, so it is also making their
diagnostics more compatible with each other.

Also, by rearranging preprocessor directives, these "unsupported" error
messages are now the only ones printed out, which is also GCC's
behaviour.

[LukeGeeson]

Very simple change, looks good to me.

Please wait a week or so before submitting, so that non-arm folks have time to raise issues

clang/utils/TableGen/NeonEmitter.cpp

OS << "/*===---- arm_neon.h - ARM Neon intrinsics "
         " *===-----------------------------------------------------------------"
         "---"
         "---===\n"
         " */\n\n";
 
   OS << "#ifndef __ARM_NEON_H\n";
   OS << "#define __ARM_NEON_H\n\n";
 
+  OS << "#ifndef __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";
+
   OS << "#if !defined(__ARM_NEON)\n";
   OS << "#error \"NEON support not enabled\"\n";
-  OS << "#endif\n\n";
+  OS << "#else\n\n";
 
   OS << "#include <stdint.h>\n\n";
 
   OS << "#ifdef __ARM_FEATURE_BF16\n";
   OS << "#include <arm_bf16.h>\n";
   OS << "typedef __bf16 bfloat16_t;\n";
   OS << "#endif\n\n";
 
while (!Defs.empty() && MadeProgress) {
     }
   }
   assert(Defs.empty() && "Some requirements were not satisfied!");
   if (!InGuard.empty())
     OS << "#endif\n";
 
   OS << "\n";
   OS << "#undef __ai\n\n";
+  OS << "#endif /* if !defined(__ARM_NEON) */\n";
+  OS << "#endif /* ifndef __ARM_FP */\n";
   OS << "#endif /* __ARM_NEON_H */\n";
 }
 
 /// run - Read the records in arm_fp16.td and output arm_fp16.h.  arm_fp16.h
 /// is comprised of type definitions and function declarations.
 void NeonEmitter::runFP16(raw_ostream &OS) {
   OS << "/*===---- arm_fp16.h - ARM FP16 intrinsics "
         "------------------------------"