[OPENMP 4.1] Parsing/sema analysis for extended format of 'if' clause.

OpenMP 4.1 added special 'directive-name-modifier' to the 'if' clause.
Format of 'if' clause is as follows:
```
if([ directive-name-modifier :] scalar-logical-expression)
```

The restriction rules are also changed.
1. If any 'if' clause on the directive includes a 'directive-name-modifier' then all 'if' clauses on the directive must include a 'directive-name-modifier'.
2. At most one 'if' clause without a 'directive-name-modifier' can appear on the directive.
3. At most one 'if' clause with some particular 'directive-name-modifier' can appear on the directive.

'directive-name-modifier' is important for combined directives and allows to separate conditions in 'if' clause for simple sub-directives in combined directive. This 'directive-name-modifier' identifies the sub-directive to which this 'if' clause must be applied.

llvm-svn: 246747