Make this more efficient of the common case where we are only mangling globals.

Do not mangle internal global names that do not collide with anything.

This gives us strings that now look like this:

__2E_str_1:                             ; '.str_1'
        .asciz  "foo"

instead of this:

l1__2E_str_1:                           ; '.str_1'
        .asciz  "foo"

llvm-svn: 24277