[MS Demangler] Fix several issues related to templates.

These were uncovered when porting the mangling tests in
ms-templates.cpp from clang/CodeGenCXX over to demangling
tests.  The main issues fixed here are surrounding integer
literal signed and unsignedness, empty array dimensions,
and pointer and reference non-type template parameters.

Differential Revision: https://reviews.llvm.org/D50512

llvm-svn: 339434