[clang][Index] Enable indexing of Template Type Parameters behind a flag

Summary:
clangd uses indexing api to provide references and it was not possible
to perform symbol information for template parameters. This patch enables
visiting of TemplateTypeParmTypeLocs.

Reviewers: ilya-biryukov, akyrtzi

Subscribers: javed.absar, kristof.beyls, ioeric, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58293

llvm-svn: 354560