On MachO, don't put non-private constants in mergeable sections.

On MachO, putting a symbol that doesn't start with a 'L' or 'l' in one of the
__TEXT,__literal* sections prevents the linker from merging the context of the
section.

Since private GVs are the ones the get mangled to start with 'L' or 'l', we now
only put those on the __TEXT,__literal* sections.

llvm-svn: 216682