GlobalValue: Assert symbols with local linkage have default visibility

The change to ExtractGV.cpp has no functionality change except to avoid
the asserts.  Existing testcases already cover this, so I didn't add a
new one.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208264 91177308-0d34-0410-b5e6-96231b3b80d8