[flang][nfc] Tweak the FrontendAction class

This patch refactors the `FrontendAction` class. It merely moves code
around so that re-using it is easier. No new functionality is
introduced.

1. Three new member methods are introduced: `RunPrescan`, `RunParse`,
`RunSemanticChecks`.
2. The following free functions are re-implemented as member methods:
  * `reportFatalSemanticErrors`
  * `reportFatalScanningErrors`
  * `reportFatalParsingErrors`
  * `reportFatalErrors`
`reportFatalSemanticErrors` is updated to resemble the other error
reporting functions and to make the API more consistent.
3. The `BeginSourceFileAction` methods are simplified and the unused
input argument is deleted.

Differential Revision: https://reviews.llvm.org/D108130