GCC_VERSION used in spirv_reflect.h but is undefined

According to [GCC's documentation](https://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html) the macro `GCC_VERSION` is not predefined, but rather can be defined as `#define GCC_VERSION (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__)` in order to compare the GCC version against a single integer constant. The `spirv_reflect.h` header uses this macro, but fails to define it causing it to be evaluated as zero and the `#if` it is used for to always be false.

Start in spirv_reflect.h and inspect the preprocessor conditional that uses GCC_VERSION alongside GCC's predefined macros. Verify the behavior with a GCC preprocessing or build check; done means the conditional compares the intended GCC version instead of treating GCC_VERSION as zero.

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