Merge #28826: ci: Switch IWYU to clang_17 branch

9f208c0 ci: Switch IWYU to `clang_17` branch (Hennadii Stepanov)

Pull request description:

  The IWYU version [0.21](https://github.com/include-what-you-use/include-what-you-use/releases/tag/0.21) has been tagged, and the `clang_17` branch is available now.

ACKs for top commit:
  maflcko:
    lgtm ACK 9f208c0

Tree-SHA512: 8b8f8743d1c2719b6383b5a6a48356ac02a301d1ce9cee77f93cc04c12de22e9ac6b59e23550a589540e68292cfac0d85bacedc9ca26f6b589011d36ee1d38cf

ci/test/01_base_install.sh

@@ -67,8 +67,7 @@ if [[ ${USE_MEMORY_SANITIZER} == "true" ]]; then
 fi
 
 if [[ "${RUN_TIDY}" == "true" ]]; then
-  ${CI_RETRY_EXE} git clone https://github.com/include-what-you-use/include-what-you-use -b master /include-what-you-use
-  git -C /include-what-you-use checkout a138eaac254e5a472464e31d5ec418fe6e6f1fc7
+  ${CI_RETRY_EXE} git clone --depth=1 https://github.com/include-what-you-use/include-what-you-use -b clang_"${TIDY_LLVM_V}" /include-what-you-use
   cmake -B /iwyu-build/ -G 'Unix Makefiles' -DCMAKE_PREFIX_PATH=/usr/lib/llvm-"${TIDY_LLVM_V}" -S /include-what-you-use
   make -C /iwyu-build/ install "-j$( nproc )"  # Use nproc, because MAKEJOBS is the default in docker image builds
 fi