Retain complete source-location information for C++

nested-name-specifiers throughout the parser, and provide a new class
(NestedNameSpecifierLoc) that contains a nested-name-specifier along
with its type-source information.

Right now, this information is completely useless, because we don't
actually store the source-location information anywhere in the
AST. Call this Step 1/N.

llvm-svn: 126391