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.

- At most one Collapse clause in SIMD construct
- A DO loop must follow the SIMD directive

Removes two XFAILed tests, the other tests are marked UNSUPPORTED only
on windows.

This fixes a compilation error when code like this is presented to the
compiler:

  !$OMP PARALLEL DO
  !DIR$ VECTOR ALIGNED
  DO 20 i=1,N
     a = a + 0.5
20   CONTINUE

The directive itself is later ignored (with a warning that this is
happening), but because the compiler already errored out before that
point, it completely fails to compile this code. Other compilers accept
the code without complaints.

For tests in the OpenMP sub-directory the `omp` prefix is removed.