Representation of template type parameters and non-type template

parameters, with some semantic analysis:
  - Template parameters are introduced into template parameter scope
  - Complain about template parameter shadowing (except in Microsoft mode)

Note that we leak template parameter declarations like crazy, a
problem we'll remedy once we actually create proper declarations for
templates. 

Next up: dependent types and value-dependent/type-dependent
expressions.

llvm-svn: 60597