[OPENMP 4.0] Support for 'uniform' clause in 'declare simd' directive.

OpenMP 4.0 defines clause 'uniform' in 'declare simd' directive:
'uniform' '(' <argument-list> ')'
The uniform clause declares one or more arguments to have an invariant value for all concurrent invocations of the function in the execution of a single SIMD loop.
The special this pointer can be used as if was one of the arguments to the function in any of the linear, aligned, or uniform clauses.

llvm-svn: 266041