[clangd] Add include-mapping for C symbols.

Summary:
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.

Reviewers: kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, jfb, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63270

llvm-svn: 364044