[OpenMP] Correctly add member expressions to OpenMP info

Mapping expressions that have `this` as their base expression aren't
considered a valid base variable and the rest of the runtime expects
this. However, if we have an expression with no value declaration we can
try to extract it manually to provide more helpful debuggin information.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D108483