[OpenMP][SYCL] Do not crash on attempt to diagnose unsupported type use

Summary:
Do not ask size of type if it is dependent. ASTContext doesn't seem expecting
this.

Reviewers: jdoerfert, ABataev, bader

Reviewed By: ABataev

Subscribers: yaxunl, guansong, ebevhan, Anastasia, sstefan1, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80829