[OpenMP] Update map clause SEMA to support OpenMP 4.5 possible list items.

Summary:
Extend support in the map clause SEMA for the expressions supported in the OpenMP 4.5 specification, namely member expressions.  

Fix some bugs in the previous implementation of SEMA related with expressions that do not consist of single variable references.

Fix bug in parsing when the expression in the map clause do not start with an identifier: accept any expression in the map clause and check for validity in SEMA instead of just ignoring it.

Reviewers: hfinkel, kkwli0, arpith-jacob, carlo.bertolli, ABataev

Subscribers: cfe-commits, fraggamuffin, caomhin

Differential Revision: http://reviews.llvm.org/D16385

llvm-svn: 258543