This patch adds support for parsing metadirectives in the C parser.

Metadirectives are represented by a OMP_METADIRECTIVE tree node. It has 
a single operand (accessed by OMP_METADIRECTIVE_CLAUSES) which contains 
a chain of TREE_LIST nodes, each one representing a clause from the 
metadirective. TREE_PURPOSE(clause) contains the selector of the clause, 
while TREE_VALUE(clause) contains another TREE_LIST - the TREE_PURPOSE 
contains the tree for the directive, while the TREE_VALUE contains the 
standalone body (if any).

If an OMP directive has an associated body, it will be part of the tree 
at TREE_PURPOSE(TREE_VALUE(clause)) - the standalone body at 
TREE_VALUE(TREE_VALUE(clause) is only used for standalone directives 
that do not have an associated body (strictly speaking, it isn't a part 
of the directive variant at all). At present, all standalone bodies in a 
metadirective are shared, and will point to the same tree node.

Labels in the statement body are handled by first scanning the body for 
labels, then enclosing the statements in a lexical block with the found 
labels declared as local using __label__. This prevents labels in the 
body interfering with each other when the body is re-parsed.

I have removed support for the 'omp begin metadirective'..'omp end 
metadirective' form of the directive that was originally in the WIP 
patch. According to the spec, the only variant directives that can be 
used in this form must have an 'end <directive>' form (apart from the 
'nothing' directive), and in C/C++, the only directive that we support 
with an end form is 'declare target', which we currently forbid since it 
is declarative.

Kwok