[lld] Allow opt-out from building lld via --skip-build-lld

[kubamracek]

rdar://126058593

[kubamracek]

@swift-ci please test

[bnbarham]

======================================================================
FAIL: test_default_value_build_lld (tests.build_swift.test_driver_arguments.TestDriverArgumentParser)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/build-user/swift/utils/build_swift/tests/build_swift/test_driver_arguments.py", line 91, in test
    self.assertEqual(default_value, parsed_value,
AssertionError: False != True : Invalid default value for "build_lld": False != True

======================================================================
FAIL: test_expected_options_exhaustive (tests.build_swift.test_driver_arguments.TestDriverArgumentParser)
Test that we are exhaustively testing all options accepted by the
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/build-user/swift/utils/build_swift/tests/build_swift/test_driver_arguments.py", line 411, in test_expected_options_exhaustive
    self.fail('non-exhaustive expected options, missing: {}'
AssertionError: non-exhaustive expected options, missing: {'--skip-build-lld'}

----------------------------------------------------------------------
Ran 1250 tests in 38.671s

FAILED (failures=2)

But otherwise 👍

[kubamracek]

@swift-ci please test