[clang] Don't use -Wno-nested-anon-types on GCC (llvm#95029)

GCC usually doesn't warn about unrecognized -Wno-<foo> options, if no
diagnostics are printed. However if some diagnostics are printed, it
also mentions that there were unrecognized -Wno-<foo> options.

Before 4feae05, we checked for whether
-Wnested-anon-types was supported, and added the -Wno-<foo> form if the
positive form of the option was supported.

As of GCC 14, -Wnested-anon-types isn't supported, thus limit the use of
the option to actual Clang (and still only while using the GCC
compatible driver).

This avoids unnecessary mentions about unrecognized -Wno-<foo> options
when building with GCC.

Author: mstorsjo

clang/CMakeLists.txt

@@ -350,7 +350,9 @@ if (LLVM_COMPILER_IS_GCC_COMPATIBLE)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pedantic -Wno-long-long")
   endif ()
 
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-nested-anon-types" )
+  if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-nested-anon-types" )
+  endif ()
 endif ()
 
 # Determine HOST_LINK_VERSION on Darwin.