Don't try to get the layout of an invalid decl in getDeclAlign.

When the decl that we're getting alignment for is a FieldDecl, and the field's
parent record is invalid, skip the actual field alignment calculation (and
return 1-byte alignment in the general case).

Also, assert in in getASTRecordLayout that the decl is valid. This was
inspired by PR16292; see also r184581 and r184751.