GV not mutated to handle for lib_6_6+

[python3kgae]

A global variable is supposed to be mutated into handle type.

But it doesn't in
tools/clang/test/HLSLFileCheck/hlsl/linker/resources/preserve_sb_types.hlsl
tools/clang/test/HLSLFileCheck/hlsl/linker/resources/preserve_cb_types.hlsl

[damyanp]

No plans to address this in DXC, label added so we can find it again when thinking about similar things in clang.