Avoid a crash after loading an #undef'd macro in code completion

In code-completion, don't assume there is a MacroInfo for everything,
since we aren't serializing the def corresponding to a later #undef in
the same module.  Also setup the HadMacro bit correctly for undefs to
avoid an assertion failure.

rdar://18416901

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218694 91177308-0d34-0410-b5e6-96231b3b80d8