[flang][driver] do not crash when fc1 process multiple files (#138875)

This is a fix for the issue https://github.com/llvm/llvm-project/issues/137126
that turned out to be a driver issue.

FrontendActions has a loop to process multiple input files and `flang -fc1`
accept multiple files, but the semantic, lowering, and llvm codegen
actions were not re-entrant, and crash or weird behaviors occurred
when processing multiple files with `-fc1`. 

This patch makes the actions reentrant by cleaning-up the contexts/modules
if needed on entry.

0 files changed, 0 insertions, 0 deletions