castxml: builds fail for 10.10/10.11: various compilation errors; blacklist older clang versions

[mascguy (Christopher Nielsen)]

The following errors are seen for 10.10/10.11, suggesting that older Xcode Clang versions need to be blacklisted:

/opt/local/libexec/llvm-15/include/llvm/ADT/STLForwardCompat.h:53:22:
error: default initialization of an object of const type 'const llvm::in_place_t' without a user-provided default constructor
constexpr in_place_t in_place; // NOLINT(readability-identifier-naming)
                     ^

/opt/local/libexec/llvm-15/include/llvm/Support/Allocator.h:238:16:
error: no viable conversion from 'long long' to 'llvm::Optional<int64_t>'
        return InSlabIdx + static_cast<int64_t>(P - S);
               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[...etc...]

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