Xcode: support gtests that use the Inputs dir

This change adds support for the gtests that require input data
in the Inputs files.  This is done through a new Xcode script
phase that runs the scripts/Xcode/prepare-gtest-run-dir.sh script.
That script simply copies the contents of all unittests/**/Inputs
dirs into ${TARGET_BUILD_DIR}/Inputs before running the test.

This change also renames the Xcode 'gtest-for-debugging' to
'gtest-build', and makes the gtest "build and run" target
depend on gtest-build.  This reduces replication within the
targets.  gtest .c/.cpp files now should only be added to
the gtest-build target.

llvm-svn: 281913