Reimplement Sema::MatchTemplateParametersToScopeSpecifier() based on

the semantic context referenced by the nested-name-specifier rather
than the syntactic form of the nested-name-specifier. The previous
incarnation was based on my complete misunderstanding of C++
[temp.expl.spec]. The latest C++0x working draft clarifies the
requirements here, and this rewrite is intended to follow that.

Along the way, improve source location information in the
diagnostics. For example, if we report that a specific type needs or
doesn't need a 'template<>' header, we dig out that type in the
nested-name-specifier and highlight its range.

Fixes: PR5907, PR9421, PR8277, PR8708, PR9482, PR9668, PR9877, and
<rdar://problem/9135379>.

llvm-svn: 131138