Fix a compilation error with LLVM 21

[alexfh]

This fixes a copy constructor must pass its first argument by reference compilation error when compiled with a recent enough Clang (after llvm/llvm-project@fe0d3e3, targeted for LLVM 21 release).

swift/stdlib/public/runtime/GenericMetadataBuilder.cpp:59:31: error: copy constructor must pass its first argument by reference
   59 |     Buffer(Buffer<const char> buffer)
      |                               ^
      |                               const &

[alexfh]

@swift-ci

[allevato]

@swift-ci please test

[al45tair]

LGTM.