[AST][RecoveryExpr] Fix an assertion crash on openMP.

Summary:
With recovery expr, it is possible that we have a value-dependent expr
within non-dependent context.

Reviewers: sammccall, jdoerfert

Subscribers: yaxunl, guansong, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80200