[IR] Add verification for module flags with the "require" behavior.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172549 91177308-0d34-0410-b5e6-96231b3b80d8
[Linker] Drop asserts that are embedded in cast<> and now checked by the verifier.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172550 91177308-0d34-0410-b5e6-96231b3b80d8