[Preprocessor] Iterating over all macros should include those from modules.

[Preprocessor] Iterating over all macros should include those from modules.

So, iterate over the list of macros mentioned in modules, and make sure those
are in the master table.

This isn't particularly efficient, but hopefully it's something that isn't
done too often.

PR23929 and rdar://problem/21480635

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@240571 91177308-0d34-0410-b5e6-96231b3b80d8