[AArch64] Alter arm_fp16.h to be target-based, not preprocessor based.

As the other recent patches, this alters the arm_fp16 intrinsics to be
target based, not preprocessor based. Apparently arm_fp16.h is AArch64
only under clang, making this mostly trivial with the TargetGuard
infrastructure.

Differential Revision: https://reviews.llvm.org/D137256

Author: davemgreen

clang/include/clang/Basic/arm_fp16.td

@@ -14,7 +14,7 @@
 include "arm_neon_incl.td"
 
 // ARMv8.2-A FP16 intrinsics.
-let ArchGuard = "defined(__ARM_FEATURE_FP16_SCALAR_ARITHMETIC) && defined(__aarch64__)" in {
+let ArchGuard = "defined(__aarch64__)", TargetGuard = "fullfp16" in {
 
   // Negate
   def VNEGSH          : SInst<"vneg", "11", "Sh">;

clang/test/Sema/aarch64-fp16-target.c

@@ -7,19 +7,19 @@
 
 __attribute__((target("fullfp16")))
 void test_fullfp16(float16_t f16) {
-  vabdh_f16(f16, f16); // expected-error {{call to undeclared function 'vabdh_f16'}}
+  vabdh_f16(f16, f16);
 }
 
 __attribute__((target("arch=armv8-a+fp16")))
 void test_fp16_arch(float16_t f16) {
-    vabdh_f16(f16, f16); // expected-error {{call to undeclared function 'vabdh_f16'}}
+    vabdh_f16(f16, f16);
 }
 
 __attribute__((target("+fp16")))
 void test_fp16(float16_t f16) {
-    vabdh_f16(f16, f16); // expected-error {{call to undeclared function 'vabdh_f16'}}
+    vabdh_f16(f16, f16);
 }
 
 void undefined(float16_t f16) {
-  vabdh_f16(f16, f16); // expected-error {{call to undeclared function 'vabdh_f16'}}
+  vabdh_f16(f16, f16); // expected-error {{'__builtin_neon_vabdh_f16' needs target feature fullfp16}}
 }