Add enumerators to TestVisitor::Language to allow visitor tests to
explicitly specify use of C++98 or C++11. Lang_CXX is preserved as
an alias for Lang_CXX98.

This does not add Lang_CXX1Y or Lang_C11, on the assumption that it's
better to add them if/when they are needed.

(This is a prerequisite for a test in a later patch for RecursiveASTVisitor.)

Reviewed by Richard Smith.