[clang] [Clang] Do not try to create invalid variable specializations. (PR #190960)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

It would have made review quicker if in the summary you said something along the lines of "modified BuildVarTemplateInstantiation so that in this case.... don't call BuildVarTemplateInstantiation and instead just diagnose and return" 

The way the context is displayed makes it difficult to see this w/o spending some time.

https://github.com/llvm/llvm-project/pull/190960