[flang] Add clang-format files for FIR source (LLVM style)

Note:
This commit does not reflect an actual work log, it is a feature based split of the
changes done in the FIR experimental branch. The related work log can be found in the commits between:
https://github.com/schweitzpgi/f18/commit/8c320e3bf2c3e9cdac66c81db3bf4634bf972e1d
and:
https://github.com/schweitzpgi/f18/commit/9b9ea05f9a75608c7bb5372c56bf7b9363569a69

Original-commit: flang-compiler/f18@00d8d5121cdaed09e584a0aea7d59baf322bfaa3
Reviewed-on: https://github.com/flang-compiler/f18/pull/959
Tree-same-pre-rewrite: false