Use the target triple from the target machine rather than the module

Use the target triple from the target machine rather than the module
to determine whether or not we're on a darwin platform for debug code
emitting.

Solves the problem of a module with no triple on the command line
and no triple in the module using non-gdb ok features on darwin. Fix
up the member-pointers test to check the correct things for cross
platform (DW_FORM_flag is a good prefix).

Unfortunately no testcase because I have no ideas how to test something
without a triple and without a triple in the module yet check
precisely on two platforms. Ideas welcome.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180660 91177308-0d34-0410-b5e6-96231b3b80d8