Fix up definitions of half and double in stdlib (#608)
Fix up definitions of half and double in stdlib (#608)

An earlier change made sure that the `half` and `double` types properly conform to the `__BuiltinFloatingPointType` and `__BuiltinRealType` interfaces, but somehow that change modified only the *generated* source file (`core.meta.slang.h`) and not the source that fed into the generator (`core.meta.slang`).
This meant that when building the compiler, we'd end up with spurious diffs because we'd run the generation logic and clobber the (correct) output file with freshly generated (wrong) code.

This change adds the missing lines to the source file to fix up the issue.
shader-slang/slang,csyonghe/slang,shader-slang/slang,csyonghe/slang,csyonghe/slang,shader-slang/slang