Fix LLVM compile error on Windows (shader-slang#4658)

The following compile error is observed when build with LLVM on Windows:
```
D:/sbf/git/slang/test_wsl/source/slang-glslang/slang-glslang.cpp:39:31: error: static assertion expression is not an integral constant expression
   39 |     SLANG_COMPILE_TIME_ASSERT(SLANG_OFFSET_OF(TBuiltInResource, limits) > 0);
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

To address the problem, SLANG_OFFSET_OF is defined as `offsetof` when
`__clang__` is defined.

Co-authored-by: Yong He <yonghe@outlook.com>

Author: jkwak-work

include/slang.h

@@ -254,15 +254,16 @@ convention for interface methods.
 
 // GCC Specific
 #if SLANG_GCC_FAMILY
-
 #	define SLANG_NO_INLINE __attribute__((noinline))
 #	define SLANG_FORCE_INLINE inline __attribute__((always_inline))
 #   define SLANG_BREAKPOINT(id) __builtin_trap();
 #	define SLANG_ALIGN_OF(T)	__alignof__(T)
+#endif // SLANG_GCC_FAMILY
 
+#if SLANG_GCC_FAMILY || defined(__clang__)
 // Use the builtin directly so we don't need to have an include of stddef.h
 #   define SLANG_OFFSET_OF(T, ELEMENT) __builtin_offsetof(T, ELEMENT) 
-#endif // SLANG_GCC_FAMILY
+#endif
 
 #ifndef SLANG_OFFSET_OF
 #   define SLANG_OFFSET_OF(T, ELEMENT) (size_t(&((T*)1)->ELEMENT) - 1)
@@ -290,10 +291,6 @@ convention for interface methods.
 #   define SLANG_COMPILE_TIME_ASSERT(x) static_assert(x)
 #endif
 
-#ifndef SLANG_OFFSET_OF
-#	define SLANG_OFFSET_OF(X, Y) offsetof(X, Y)
-#endif
-
 #ifndef SLANG_BREAKPOINT
 // Make it crash with a write to 0!
 #   define SLANG_BREAKPOINT(id) (*((int*)0) = int(id));