[flang] Make extension explicit: exponent-letter matching kind-param

As an extension for REAL literals, we allow an exponent letter which
matches an explicit kind-param. The standard requires the exponent
to be 'E' if a kind-param is present. This patch
 - documents this extension in Extensions.md
 - enables a portability warning if it is used with -pedantic

The test case for this, kinds05.f90, needs D125804, which makes
test_errors.py test warnings as well, to actually test the warnings.
I include it already now to keep things together, it will do no harm
(I hope ...).

We also add WARNING-directives to the test kinds04.f90 in preparation
for D125804. As the exponent-letter 'Q' does not imply the same kind
on all platforms, the emitted warnings are platform-dependent.
Therefore, the test is duplicated into two variants which are run
conditionally.

Finally, we promote the portability warning for when the exponent letter
is neither 'E' nor matching the kind-param to a standard warning.

Reviewed By: klausler

Differential Revision: https://reviews.llvm.org/D126459