Renamed Lang_C to Lang_C99, Lang_CXX to Lang_CXX03, and 2a to 20

Summary:
I think we would be better off with tests explicitly specifying the
language mode. Right now Lang_C means C99, but reads as "any C version",
or as "unspecified C version".

I also changed '-std=c++98' to '-std=c++03' because they are aliases (so
there is no difference in practice), because Clang implements C++03
rules in practice, and because 03 makes a nice sortable progression
between 03, 11, 14, 17, 20.

Reviewers: shafik, hlopko

Reviewed By: hlopko

Subscribers: jfb, martong, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81000