[libc] Define LLVM_LIBC_FUNCTION with a layer of macro expansion

[mcgrathr]

Summary

Move the real LLVM_LIBC_FUNCTION macro definitions to
LLVM_LIBC_FUNCTION_IMPL and make LLVM_LIBC_FUNCTION a wrapper to
expand macros in its arguments. This makes it possible to
compile libc implementation and test files with -Dfunc=othername.

libc/src/__support/common.h

 #include "src/__support/macros/properties/architectures.h"
 
 #ifndef LLVM_LIBC_FUNCTION_ATTR
 #define LLVM_LIBC_FUNCTION_ATTR
 #endif
 
 // GPU targets do not support aliasing.
 #if defined(LIBC_COPT_PUBLIC_PACKAGING) && defined(LIBC_TARGET_ARCH_IS_GPU)
-#define LLVM_LIBC_FUNCTION(type, name, arglist)                                \
+#define LLVM_LIBC_FUNCTION_IMPL(type, name, arglist)                           \
   LLVM_LIBC_FUNCTION_ATTR decltype(__llvm_libc::name)                          \
       __##name##_impl__ __asm__(#name);                                        \
   type __##name##_impl__ arglist
 // MacOS needs to be excluded because it does not support aliasing.
 #elif defined(LIBC_COPT_PUBLIC_PACKAGING) && (!defined(__APPLE__))
-#define LLVM_LIBC_FUNCTION(type, name, arglist)                                \
+#define LLVM_LIBC_FUNCTION_IMPL(type, name, arglist)                           \
   LLVM_LIBC_FUNCTION_ATTR decltype(__llvm_libc::name)                          \
       __##name##_impl__ __asm__(#name);                                        \
   decltype(__llvm_libc::name) name [[gnu::alias(#name)]];                      \
   type __##name##_impl__ arglist
 #else
-#define LLVM_LIBC_FUNCTION(type, name, arglist) type name arglist
+#define LLVM_LIBC_FUNCTION_IMPL(type, name, arglist) type name arglist
 #endif
 
+// This extra layer of macro allows `name` to be a macro to rename a function.
+#define LLVM_LIBC_FUNCTION(type, name, arglist)                                \
+  LLVM_LIBC_FUNCTION_IMPL(type, name, arglist)
+
 namespace __llvm_libc {
 namespace internal {
 constexpr bool same_string(char const *lhs, char const *rhs) {
   for (; *lhs || *rhs; ++lhs, ++rhs)
     if (*lhs != *rhs)
       return false;
   return true;
 }