[SemaObjC] Do not RebuildObjCMessageExpr without valid method decl

Fix crash-on-invalid in ObjC Sema by avoiding to rebuild a message
expression to a 'super' class in case the method to call does not exist
(i.e. comes from another missing identifier).

In this case, the typo transform is invoked upon the message expression
in an attempt to solve a typo in a 'super' call parameters, but it
crashes since it assumes the method to call has a valid declaration.

rdar://problem/27305403

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@279481 91177308-0d34-0410-b5e6-96231b3b80d8