[CodeGen] Fix a crash that occurs when a non-trivial C struct with a

volatile array field is copied.

The crash occurs because method 'visitArray' passes a null FieldDecl to
method 'visit' and some of the methods called downstream expect a
non-null FieldDecl to be passed.

This reapplies r330151 with a fix to the test case.

rdar://problem/33599681

llvm-svn: 330155