Make global aliases have symbol size equal to their type

Make global aliases have symbol size equal to their type

This is mainly for the benefit of GlobalMerge, so that an alias into a
MergedGlobals variable has the same size as the original non-merged
variable.

Differential Revision: http://reviews.llvm.org/D10837

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242520 91177308-0d34-0410-b5e6-96231b3b80d8