[libclang] Don't disable the preprocessing record while indexing so as

[libclang] Don't disable the preprocessing record while indexing so as
to not mess up with module building.

It was not worth trying to combine indexing without preprocessing record
and building modules with one because:

-just importing a module/PCH that was built with a pp record, enables it anyway
-the performance gain of indexing without the preprocessing record is insignificant.

llvm-svn: 165352