[flang] Produce proper "preprocessor output" for -E option
Rename the current -E option to "-E -Xflang -fno-reformat".
Add a new Parsing::EmitPreprocessedSource() routine to convert the
cooked character stream output of the prescanner back to something
more closely resembling output from a traditional preprocessor;
call this new routine when -E appears.
The new -E output is suitable for use as fixed form Fortran source to
compilation by (one hopes) any Fortran compiler. If the original
top-level source file had been free form source, the output will be
suitable for use as free form source as well; otherwise there may be
diagnostics about missing spaces if they were indeed absent in the
original fixed form source.
Unless the -P option appears, #line directives are interspersed
with the output (but be advised, f18 will ignore these if presented
with them in a later compilation).
An effort has been made to preserve original alphabetic character case
and source indentation.
Add -P and -fno-reformat to the new drivers.
Tweak test options to avoid confusion with prior -E output; use
-fno-reformat where needed, but prefer to keep -E, sometimes
in concert with -P, on most, updating expected results accordingly.