Semantic checking for class template declarations and

redeclarations. For example, checks that a class template
redeclaration has the same template parameters as previous
declarations.

Detangled class-template checking from ActOnTag, whose logic was
getting rather convoluted because it tried to handle C, C++, and C++
template semantics in one shot.

Made some inroads toward eliminating extraneous "declaration does not
declare anything" errors by adding an "error" type specifier.

llvm-svn: 63973