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.

Author: rjmccall

include/swift/Remote/MetadataReader.h

@@ -1353,7 +1353,10 @@ class MetadataReader {
       if (IsaMagicMask != 0) {
         tryFindAndReadSymbol(IsaMagicValue,
                              "objc_debug_indexed_isa_magic_value");
-        tryFindAndReadSymbol(IsaMask, "objc_debug_indexed_isa_index_mask");
+        tryFindAndReadSymbol(IsaIndexMask,
+                             "objc_debug_indexed_isa_index_mask");
+        tryFindAndReadSymbol(IsaIndexShift,
+                             "objc_debug_indexed_isa_index_shift");
         tryFindSymbol(indexedClasses, "objc_indexed_classes");
         IndexedClassesPointer = indexedClasses.getAddressData();
         tryFindSymbol(indexedClassesCount, "objc_indexed_classes_count");