[clang][ARM] Add name-mangling test for direct __fp16 arguments.

[simon_tatham]

Summary

clang/test/CodeGenCXX/fp16-mangle.cpp tests pointers to fp16, but
if you give the -fallow-half-arguments-and-returns option, then
clang can also leave an fp16 unmodified as a function argument or
return type. This regression test checks the name-mangling of that.

[miyuki]

The mangling agrees with https://itanium-cxx-abi.github.io/cxx-abi/abi.html#mangling-builtin
LGTM

clang/test/CodeGenCXX/fp16-mangle-arg-return.cpp

+// RUN: %clang_cc1 -emit-llvm -o - -triple arm-arm-none-eabi -fallow-half-arguments-and-returns %s | FileCheck %s
+// RUN: %clang_cc1 -emit-llvm -o - -triple aarch64-arm-none-eabi -fallow-half-arguments-and-returns %s | FileCheck %s
+
+// Test name-mangling of __fp16 passed directly as a function argument
+// (when that is permitted).
+
+// CHECK: define {{.*}}void @_Z13fp16_argumentDh(half %{{.*}})
+void fp16_argument(__fp16 arg) {}
+
+// Test name-mangling of __fp16 as a return type. The return type of
+// fp16_return itself isn't mentioned in the mangled name, so to test
+// this, we have to pass it a function pointer and make __fp16 the
+// return type of that.
+
+// CHECK: define {{.*}}void @_Z11fp16_returnPFDhvE(half ()* %{{.*}})
+void fp16_return(__fp16 (*func)(void)) {}