[OPENMP50] Fix PR45469: Consider variable-category of defaultmap clause as optional.

Summary:
According to the standard, variable-category is the optional part of the
defaultmap clause while the compiler always requires it. Turned it into
optional part.

Reviewers: jdoerfert

Subscribers: yaxunl, guansong, cfe-commits, caomhin

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77751