Special case aliases in GlobalValue::getSection.

This is similar to the getAlignment patch, but is done just for
completeness. It looks like we never call getSection on an alias. All the
tests still pass if the if is replaced with an assert.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208139 91177308-0d34-0410-b5e6-96231b3b80d8