`TokenSequence::pop_back()` had a check assumed that tokens are never
empty. Loosen this check since isn't true.

towards #146362

Address failing Fujitsu test suite cases that were broken by the patch
to defer the handling of !$ lines in -fopenmp vs. normal compilation to
actual compilation rather than processing them immediately in -E mode.

Tested on the samples in the bug report as well as all of the Fujitsu
tests that I could find that use !$ lines.

Fixes https://github.com/llvm/llvm-project/issues/136845.

See new test. I inadvertently broke this behavior with a recent fix for
another problem, because the effects of the overloaded
TokenSequence::Put() member function on token merging were confusing.
Rename and document the various overloads.

For numeric kind suffixes like 1_foo, the preprocessor should be able to
perform macro replacement for macros named either "_foo" or "foo".

Fixes https://github.com/llvm/llvm-project/issues/133399.

No longer require -fopenmp or -fopenacc with -E, unless specific version
number options are also required for predefined macros. This means that
most source can be preprocessed with -E and then later compiled with
-fopenmp, -fopenacc, or neither.

This means that OpenMP conditional compilation lines (!$) are also
passed through to -E output. The tricky part of this patch was dealing
with the fact that those conditional lines can also contain regular
Fortran line continuation, and that now has to be deferred when !$ lines
are interspersed.

The preprocessor implements "defined(X)" and "defined X" in if/elif
directive expressions in such a way that they only work at the top
level, not when they appear in macro expansions. Fix that, which is a
little tricky due to the need to detect the "defined" keyword before
applying any macro expansion to its argument, and add a bunch of tests.

Fixes https://github.com/llvm/llvm-project/issues/114064.

Don't treat !DIR$ or an active !$ACC, !$OMP, &c. as a comment when they
appear after a semicolon, but instead treat them as a compiler directive
sentinel.

…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.

…e produced by keyword macro replacement

When later initial keyword macro replacement will yield a line that is
not Fortran source, don't interpret "&" as a Fortran source line
continuation marker during tokenization of the line.

Fixes https://github.com/llvm/llvm-project/issues/82579.

This should work the same way as in clang.