[flang][OpenMP] Implement loop nest parser (#168884)
Previously, loop constructs were parsed in a piece-wise manner: the
begin directive, the body, and the end directive were parsed separately.
Later on in canonicalization they were all coalesced into a loop
construct. To facilitate that end-loop directives were given a special
treatment, namely they were parsed as OpenMP constructs. As a result
syntax errors caused by misplaced end-loop directives were handled
differently from those cause by misplaced non-loop end directives.
The new loop nest parser constructs the complete loop construct,
removing the need for the canonicalization step. Additionally, it is the
basis for parsing loop-sequence-associated constructs in the future.
It also removes the need for the special treatment of end-loop
directives. While this patch temporarily degrades the error messaging
for misplaced end-loop directives, it enables uniform handling of any
misplaced end-directives in the future.
