[JITLink] Make sure we explicitly deallocate memory on failure.

JITLinkGeneric phases 2 and 3 (focused on applying fixups and finalizing memory,
respectively) may fail for various reasons. If this happens, we need to
explicitly de-allocate the memory allocated in phase 1 (explicitly, because
deallocation may also fail and so is implemented as a method returning error).

No testcase yet: I am still trying to decide on the right way to test totally
platform agnostic code like this.

llvm-svn: 359643