Implement C++ [temp.local]p8, which specifies that a template

parameter hides a namespace-scope declararion with the same name in an
out-of-line definition of a template. The lookup requires a strange
interleaving of lexical and semantic scopes (go C++), which I have not
yet handled in the typo correction/code completion path.

Fixes PR6594.

llvm-svn: 98544