Revert r207477 (and r207479), "Check -Werror options during module validation"

It tried to introduce cyclic dependencies. Serialization shouldn't depend on Frontend, since Frontend depends on Serialization.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207497 91177308-0d34-0410-b5e6-96231b3b80d8