[clangd] Add include-mapping for C symbols.
This resolves the issue of introducing c++-style includes for C files.
- refactor the gen_std.py, make it reusable for parsing C symbols.
- add a language mode to the mapping method to use different mapping for C and C++ files.
Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, jfb, cfe-commits
Differential Revision: https://reviews.llvm.org/D63270