If a macro has been #undef'd in a precompiled header, we still need to

write out the macro history for that macro. Similarly, we need to cope
with reading a macro definition that has been #undef'd.

Take advantage of this new ability so that global code-completion
results can refer to #undef'd macros, rather than losing them
entirely. For multiply defined/#undef'd macros, we will still get the
wrong result, but it's better than getting no result.

llvm-svn: 165502