This patch contains the required support for metadirectives in the 
middle-end.

The tree metadirective representation is gimplified into the high Gimple 
representation, which is structured like this:

#pragma omp metadirective
   when (<selector1>):
     <directive body 1>
     goto body_label|end_label
   when (<selector2>>:
     <directive body 2>
     goto body_label|end_label
   default:
     <directive body N>
     goto body_label|end_label
body_label:
   <standalone body>
end_label:

Each variant ends with an explicit goto to either the shared standalone 
body (if the variant uses it) or to the point after the body (if it does 
not).

When lowered to low Gimple, the directive bodies move outside of the 
metadirective statement, retaining only the labels to the bodies, so it 
looks like this instead:

#pragma omp metadirective
   when (<selector1>): goto body1_label
   when (<selector2>>: goto body2_label
   default: goto default_label
body1_label:
   <directive body 1>
   goto body_label|end_label
body2_label:
   <directive body 2>
   goto body_label|end_label
default_label:
   <directive body N>
   goto body_label|end_label
body_label:
   <standalone body>
end_label:

When scanning the OpenMP regions in the ompexp pass, we create a 'clone' 
of the surrounding context when recursively scanning the directive 
variants. If the same outer context was used for all variants, then it 
would appear as if all the variants were inside the region at the same 
time (only one variant of the metadirective is ever active at a time), 
which can lead to spurious errors.

The rest of the code is the plumbing required to allow the Gimple 
metadirective statement to pass through the middle-end. GCC will emit an 
ICE if it makes it through to the back-end though, as the metadirective 
is supposed to be eliminated before it gets that far.

Kwok