Redo "Make global aliases have symbol size equal to their type"

Redo "Make global aliases have symbol size equal to their type"

r242520 was reverted in r244313 as the expected behaviour of the alias
attribute in C is that the alias has the same size as the aliasee. However
we can re-introduce adding the size on the alias when the aliasee does not,
from a source code or object perspective, exist as a discrete entity. This
happens when the aliasee is not a symbol, or when that symbol is private.

Differential Revision: http://reviews.llvm.org/D11943

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@244752 91177308-0d34-0410-b5e6-96231b3b80d8