Refactor and generalize GetAddrOfConstantString and GetAddrOfConstantStringFromLiteral.

Share mode code between these functions and re-structure them in a way
which shows how similar they actually are. The latter function works well
with literals of multi-byte chars and does a GlobalVariable name mangling
(if global strings are non-writable).

No functionality change.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210212 91177308-0d34-0410-b5e6-96231b3b80d8