Revert r237609 for now.

glibc's headers use __need_* macros to selectively export parts of themselves
to each other. This requires us to enter those files repeatedly when building
a glibc module.

This can be unreverted once we have a better mechanism to deal with that
non-modular aspect of glibc (possibly some way to mark a header as "textual if
this macro is defined").

llvm-svn: 237718