[flang] Semantic checks to see if a DO variable is modified

[flang] Semantic checks to see if a DO variable is modified

I added infrastructure to SemanticsContext to track active DO variables
and the source locations where they appear in DO statements.  I also
added code to semantics.[h,cc] to check to see if a DO variable is
already defined, and, if so, to emit an error message along with a
reference to the relevant DO construct.  I also added calls to several
places where variables are defined to determine if the definitions are
happening in the context of an active DO construct.

I have not yet added the checks for DO variables being redefined when passing
them as actual arguments to dummy arguments with INTENT(OUT) or INTENT(INOUT).
I wanted to get these changes merged first and catch up with the other changes
in master.

Original-commit: flang-compiler/f18@1bbfcca61b94dccfbbe943b511f846acc1c33997
Reviewed-on: https://github.com/flang-compiler/f18/pull/860