[clang] Add option to clear AST memory before running LLVM passes
This is to save memory for Clang compiles.
Measuring building PassBuilder.cpp under /usr/bin/time, max rss goes from 0.93GB to 0.7GB.
This does not turn it by default yet.
I've turned on the option locally and run it over a good amount of files without any issues.
For more background, see
Reviewed By: dblaikie
Differential Revision: https://reviews.llvm.org/D111105