Naturally align doubles in the constant pool, set PrivateGlobalPrefix on

Naturally align doubles in the constant pool, set PrivateGlobalPrefix on
darwin, use it when printing the constant pool indices so the labels are
appropriately private, emit cp entries to .const instead of .data on darwin
and only emit a single .section for the constant pool, not one for each
entry.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@24440 91177308-0d34-0410-b5e6-96231b3b80d8