[flang] Support correct continuations for compiler directives

If a line is over 72 characters long, flang's preprocessor cuts it there
and continues on the next line.
For this purpose it uses the standard way of continuing line with & on each line.
However, it doesn't work with long compiler directives, like OpenMP or OpenACC ones.
The line that continues the directive also has to
contain the corresponding sentinel at the beginning.

This change implements the described functionality.
Also, some code was refactored in order to simplify and reuse existing code.

Reviewed By: klausler

Differential Revision: https://reviews.llvm.org/D126301