[clangd] Fixed printTemplateSpecializationArgs not printing partial variable...

[clangd] Fixed printTemplateSpecializationArgs not printing partial variable specialization arguments.

Summary:
printTemplateSpecializationArgs was not printing partial variable specialization args. This adds an additional If clause where we check if it's a VariableTemplatePartialSpecializationDecl and returns the ArgumentLocs if that's the case.
Also adds tests for printTemplateSpecializationArgs in ASTTests.cpp.

Reviewers: hokein, ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65926

llvm-svn: 368403