[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.

Differential Revision: https://reviews.llvm.org/D106727