Fix the indexed-ISA metadata reader by properly initializing IsaIndexShift.
Fix the indexed-ISA metadata reader by properly initializing IsaIndexShift.

Unfortunately, I'm flying blind a bit --- this code is hard to
test outside of an integrated environment on a target using the
ABI in question, and I don't have such an environment set up ---
so I'm left with this rather ugly process of iteratively submitting
patches for trivial bugs and waiting for the bots to deliver judgment.
The environment could be mocked with a test environment that used
a nonstandard ObjC ABI, but that would actually take a lot of work
because of Swift's assumptions about the ObjC ABI: we'd basically
have to configure the test environment as a real, proper target,
possibly all the way through LLVM.

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