[index] Fix crash when indexing a C++14 PCH/module related to TemplateTemplateParmDec...

[index] Fix crash when indexing a C++14 PCH/module related to TemplateTemplateParmDecls of alias templates

TemplateTemplateParmDecls of alias templates ended-up serialized as 'file-level decls' which was causing a crash while trying to index a PCH/module file that contained them.
Commit makes sure TemplateTemplateParmDecls are not recorded as such kind of decls.

Fixes crash of rdar://36608297

Differential Revision: https://reviews.llvm.org/D42588

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@323549 91177308-0d34-0410-b5e6-96231b3b80d8