[clang][IFS] Adds support for more decl types in clang interface stubs.

Adding support for processing the following Decls: NonTypeTemplateParmDecl,
CXXConversionDecl, UnresolvedUsingValueDecl, UsingDecl, UsingShadowDecl,
TypeAliasTemplateDecl, TypeAliasDecl, VarTemplateDecl,
VarTemplateSpecializationDecl, UsingDirectiveDecl, TemplateTemplateParmDecl,
ClassTemplatePartialSpecializationDecl, IndirectFieldDecl.

Also, this allows for processing NamedDecls that don't have an identifier and
skips over VarDecls that are dependent on template types.

Differential Revision: https://reviews.llvm.org/D69995