Change the GlobalAlias constructor to look a bit more like GlobalVariable.

This is part of the fix for pr10367. A GlobalAlias always has a pointer type,
so just have the constructor build the type.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208983 91177308-0d34-0410-b5e6-96231b3b80d8