[Sema] Produce diagnostics for attribute 'trivial_abi' that appears

after the closing brace of a class declaration.

Merge the two call sites of checkIllFormedTrivialABIStruct and sink it
into CheckCompletedCXXClass so that it is called after the attribute has
been attached to the CXXRecordDecl.

rdar://problem/40873297

llvm-svn: 335021