Cygwin and MinGW should use the autolink feature in the sameway of Linux
due to the linker's limit. Now swift-autolink-extract recognizes the
COFF format file for Cygwin/MinGW.
Rather than relying on the system to provide ar and nm, use the LLVM equivalent
tools instead. Since the build is already dependent on llvm and its tools, this
just ensures that the full set is self-contained. NFC.
Previously it just gave up when it saw anything other than an ELF .o file. We
could have it ignore .a files, but we might as well do the right thing.
<rdar://problem/23045632>
