[flang][preprocessor] Fixed-form continuation across preprocessing di… (#95332)

…rective

Implement fixed-form line continuation when the continuation line is the
result of text produced by an #include or other preprocessing directive.
This accommodates the somewhat common practice of putting dummy or
actual arguments into a header file and #including it into several code
sites.

Fixes https://github.com/llvm/llvm-project/issues/78928.