[AST][RecoveryExpr] Fix a crash on a field decl with invalid type.

Summary:
The field decl (in the testcase) was still valid, which results in a
valid RecordDecl, it led to crash when performing struct layout,
and computing struct size etc.

Reviewers: sammccall

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81913