build: Bump compilation toolchain

[varungandhi-src]

Previously, we were using Clang 15.0.6 which had a bug
that would trigger when compiling Clang HEAD in C++20 mode.

external/llvm-project/clang/lib/Analysis/FlowSensitive/HTMLLogger.cpp:503:12: error: ISO C++20 does not permit initialization of char array with UTF-8 string literal
      char ConvergenceMarker[] = u8"\\n\u2192\u007c";
           ^

This prevented us from bumping the LLVM version we
build/link against. So bump Clang so that we can bump Clang.

[varungandhi-src]

Current CI failure is because of llvm/llvm-project#75490

Slack discussion: https://sourcegraph.slack.com/archives/C04MYFW01NV/p1709292157197109

[varungandhi-src]

Another CI failure now:

(01:34:03) ERROR: /buildkite/builds/buildkite-agent-bazel-667dfd7c7b-ts77f-1/sourcegraph/scip-clang/indexer/BUILD:3:11: Compiling indexer/Preprocessing.cc failed: (Exit 1): cc_wrapper.sh failed: error executing command (from target //indexer:scip-clang-lib) external/llvm_toolchain/bin/cc_wrapper.sh -U_FORTIFY_SOURCE '--target=x86_64-unknown-linux-gnu' -U_FORTIFY_SOURCE -fstack-protector -fno-omit-frame-pointer -fcolor-diagnostics -Wall -Wthread-safety ... (remaining 527 arguments skipped)
Use --sandbox_debug to see verbose messages from the sandbox and retain the sandbox build root for debugging
In file included from indexer/Preprocessing.cc:15:
./indexer/Indexer.h:83:3: error: defaulting this equality comparison operator is not allowed because it was already declared outside the class
  DERIVE_EQ_ALL(FileLocalSourceRange)
  ^
./indexer/Derive.h:16:15: note: expanded from macro 'DERIVE_EQ_ALL'
  friend bool operator==(const _Type &, const _Type &) = default;
              ^
./indexer/Indexer.h:82:3: note: previous declaration is here
  DERIVE_CMP_ALL(FileLocalSourceRange)
  ^
./indexer/Derive.h:27:3: note: expanded from macro 'DERIVE_CMP_ALL'
  DERIVE_EQ_ALL(_Type)                                                    \
  ^
./indexer/Derive.h:16:15: note: expanded from macro 'DERIVE_EQ_ALL'
  friend bool operator==(const _Type &, const _Type &) = default;
              ^
1 error generated.