The official languages that OpenMP recognizes are C/C++ and Fortran.
Some OpenMP directives are language-specific, some are C/C++-only, some
are Fortran-only.
Add a property to the TableGen definition of Directive that will be the
list of languages that allow the directive.
The TableGen backend will then generate a bitmask-like enumeration
SourceLanguages, and a function
SourceLanguages getDirectiveLanguages(Directive D);
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