[Sema] Add implicit members even for invalid CXXRecordDecls

Summary:
It should be safe, since other code paths are already generating
implicit members even in invalid CXXRecordDecls (e.g. lookup).

If we don't generate implicit members on CXXRecordDecl's completion,
they will be generated by next lookup of constructors. This causes a
crash when the following conditions are met:
  - a CXXRecordDecl is invalid,
  - it is provided via ExternalASTSource (e.g. from PCH),
  - it has inherited constructors (they create ShadowDecls),
  - lookup of its constructors was not run before ASTWriter serialized
    it.

This may require the ShadowDecls created for inherited constructors to
be removed from the class, but that's no longer possible since class is
provided by ExternalASTSource.

See provided lit test for an example.

Reviewers: bkramer

Reviewed By: bkramer

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D42810

llvm-svn: 324062