Diagnose the various invalid decl-specifiers on nontype template parameters.
The standard correctly forbids various decl-specifiers that dont make sense on non-type template parameters - such as the extern in:
template<extern int> struct X;
This patch implements those restrictions (in a fashion similar to the corresponding checks on function parameters within ActOnParamDeclarator).
Credit goes to miyuki (Mikhail Maltsev) for drawing attention to this issue, authoring the initial versions of this patch, and supporting the effort to re-engineer it slightly. Thank you!
For details of how this patch evolved please see: https://reviews.llvm.org/D40705