[clang-extdef-mapping] Directly process .ast files

When doing CTU analysis setup you pre-compile .cpp to .ast and then
you run clang-extdef-mapping on the .cpp file as well. This is a
pretty slow process since we have to recompile the file each time.

With this patch you can now run clang-extdef-mapping directly on
the .ast file. That saves a lot of time.

I tried this on llvm/lib/AsmParser/Parser.cpp and running
extdef-mapping on the .cpp file took 5.4s on my machine.

While running it on the .ast file it took 2s.

This can save a lot of time for the setup phase of CTU analysis.

Reviewed By: martong

Differential Revision: https://reviews.llvm.org/D128704