[Flang][OpenMP][Lower] Refactor implementation of PFT to MLIR lowering

This patch makes the following non-functional changes:
  - Extract OpenMP clause processing into a new internal `ClauseProcessor`
    class. Atomic and reduction-related clauses processing is kept unchanged,
    since atomic clauses are stored in `OmpAtomicClauseList` rather than
    `OmpClauseList` and there are many TODO comments related to the current
    implementation of reduction lowering. This has been left unchanged to avoid
    merge conflicts and work duplication.
  - Reorganize functions into sections in the file to improve readability.
  - Explicitly use mlir:: namespace everywhere, rather than just most places.
  - Spell out uses of `auto` in which the type wasn't explicitly stated as part
    of the initialization expression.
  - Normalize a few function names to match the rest and renamed variables in
    'snake_case' to 'camelCase'.

The main purpose is to reduce code duplication and simplify the implementation
of upcoming work to support loop-attached target constructs and teams/
distribute lowering to MLIR.

Differential Revision: https://reviews.llvm.org/D155981