[OPENMP51]Initial parsing/sema for append_args clause for 'declare variant'

[OPENMP51]Initial parsing/sema for append_args clause for 'declare variant'

Adds initial parsing and sema for the 'append_args' clause.

Note that an AST clause is not created as it instead adds its values
to the OMPDeclareVariantAttr.

Differential Revision: https://reviews.llvm.org/D111854