Fix the '-fuse-init-array' option to actually be an option.

Previously, this flag to CC1 was never exposed at the clang driver
layer, and if you happened to enable it (by being on Android or GCC 4.7
platform), you couldn't *disable* it, because there was no 'no' variant.
The whole thing was confusingly implemented.

Now, the target-specific flag processing gets the driver arg list, and
we use standard hasFlag with a default based on the GCC version and/or
Android platform. The user can still pass the 'no-' variant to forcibly
disable the flag, or pass the positive variant to clang itself to enable
the flag.

The test has also been substantially cleaned up and extended to cover
these use cases.

llvm-svn: 168473