CodeGen: tweak CFConstantStrings for COFF and ELF

Adjust the constant CFString emission to emit into more appropriate sections on
ELF and COFF targets.  It would previously try to use MachO section names
irrespective of the file format.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@271211 91177308-0d34-0410-b5e6-96231b3b80d8