Improve support for Clang inside of UPB (#17433)

After using UPB I noticed that field accessors were not getting inlined properly using Clang. The source appears to be the UPB_FORCEINLINE macro falling back to just being `static` when underneath clang despite clang having full support of the required attributes.

Closes #17433

COPYBARA_INTEGRATE_REVIEW=#17433 from Vadmeme:main 57204e7
PiperOrigin-RevId: 699195477

Author: Vadmeme

upb/port/def.inc

@@ -132,7 +132,7 @@ Error, UINTPTR_MAX is undefined
 #endif
 
 // Macros for function attributes on compilers that support them.
-#ifdef __GNUC__
+#if defined(__GNUC__) || defined(__clang__)
 #define UPB_FORCEINLINE __inline__ __attribute__((always_inline)) static
 #define UPB_NOINLINE __attribute__((noinline))
 #define UPB_NORETURN __attribute__((__noreturn__))