Implement "incremental" template instantiation for non-type template

Implement "incremental" template instantiation for non-type template
parameters and template type parameters, which occurs when
substituting into the declarations of member templates inside class
templates. This eliminates errors about our inability to "reduce
non-type template parameter depth", fixing PR5311.

Also fixes a bug when instantiating a template type parameter
declaration in a member template, where we weren't properly reducing
the template parameter's depth.

LLVM's StringSwitch header now parses.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@85669 91177308-0d34-0410-b5e6-96231b3b80d8