[Tooling] Add a utility function to replace one nested name with another.

One problem in clang-tidy and other clang tools face is that there is no
way to lookup an arbitrary name in the AST, that's buried deep inside Sema
and might not even be what the user wants as the new name may be freshly
inserted and not available in the AST.

A common use case for lookups is replacing one nested name with another
while minimizing namespace qualifications, so replacing 'ns::foo' with
'ns::bar' will use just 'bar' if we happen to be inside the namespace 'ns'.
This adds a little helper utility for exactly that use case.

Differential Revision: http://reviews.llvm.org/D13931

llvm-svn: 251022