[libc++][NFC] Refactor _LIBCPP_OVERRIDABLE_FUNCTION to be a normal attribute macro (#174964)

Currently `_LIBCPP_OVERRIDABLE_FUNCTION` takes the return type, function
name and argument list, but simply constructs the function and adds
attributes without modifying the signature in any way. We can replace
this with a normal attribute macro, making the signature easier to read
and simpler to understand what's actually going on. Since it's an
internal macro we can also drop the `_LIBCPP_` prefix.

NOKEYCHECK=True
GitOrigin-RevId: a5cab90071fbf6f700c95c337d850845ccc9ed9b

src/stdlib_new_delete.cpp

diff --git a/src/stdlib_new_delete.cpp b/src/stdlib_new_delete.cpp
index dbb75b1..783357c 100644
--- a/src/stdlib_new_delete.cpp
+++ b/src/stdlib_new_delete.cpp
@@ -63,7 +63,7 @@
   return p;
 }
 
-_LIBCPP_OVERRIDABLE_FUNCTION(void*, operator new, (std::size_t size)) _THROW_BAD_ALLOC {
+OVERRIDABLE_FUNCTION void* operator new(std::size_t size) _THROW_BAD_ALLOC {
   void* p = operator_new_impl(size);
   if (p == nullptr)
     __throw_bad_alloc_shim();
@@ -94,7 +94,7 @@
 #endif
 }
 
-_LIBCPP_OVERRIDABLE_FUNCTION(void*, operator new[], (size_t size)) _THROW_BAD_ALLOC { return ::operator new(size); }
+OVERRIDABLE_FUNCTION void* operator new[](size_t size) _THROW_BAD_ALLOC { return ::operator new(size); }
 
 _LIBCPP_WEAK void* operator new[](size_t size, const std::nothrow_t&) noexcept {
 #if !_LIBCPP_HAS_EXCEPTIONS
@@ -154,7 +154,7 @@
   return p;
 }
 
-_LIBCPP_OVERRIDABLE_FUNCTION(void*, operator new, (std::size_t size, std::align_val_t alignment)) _THROW_BAD_ALLOC {
+OVERRIDABLE_FUNCTION void* operator new(std::size_t size, std::align_val_t alignment) _THROW_BAD_ALLOC {
   void* p = operator_new_aligned_impl(size, alignment);
   if (p == nullptr)
     __throw_bad_alloc_shim();
@@ -185,7 +185,7 @@
 #  endif
 }
 
-_LIBCPP_OVERRIDABLE_FUNCTION(void*, operator new[], (size_t size, std::align_val_t alignment)) _THROW_BAD_ALLOC {
+OVERRIDABLE_FUNCTION void* operator new[](size_t size, std::align_val_t alignment) _THROW_BAD_ALLOC {
   return ::operator new(size, alignment);
 }