Mark invalid RecordDecls as completed.

Sema::ActOnTag creates TagDecls for records. However, if those record
declarations are invalid, and the parser is in C++ mode, it would
silently drop the TagDecl (and leave it as "beingDefined"). The problem
is that other code (e.g. the ASTWriter) will serialize all types, and
expects them to be complete. So, leaving them open would result in
failing asserts.

Fixes PR20320

Differential Revision: http://reviews.llvm.org/D21176

llvm-svn: 285275