Update Clang toolchain from 18.0.0 to 18.1.8

This is done in the interest of assisting google#12075 and google#11626. Currently
the Rust toolchain cannot be updated because the latest nightly uses
LLVM 18.1.8 and coverage information breaks. This breakage is because
LLVM 18.1.8 records coverage information with version "9" but LLVM
18.0.0 recorded coverage information with version "8". This means that
the recordings created by Rust binaries use version "9" which are
unreadable by the processing that OSS-Fuzz does with the 18.0.0-based
toolchain using version "8".

This commit updates the Clang toolchain to the latest 18.x.x release to
get the two in sync so the same coverage recording version is used.

infra/base-images/base-clang/checkout_build_install_llvm.sh

@@ -54,7 +54,7 @@ apt-get update && apt-get install -y $LLVM_DEP_PACKAGES --no-install-recommends
 # languages, projects, ...) is needed.
 # Check CMAKE_VERSION infra/base-images/base-clang/Dockerfile was released
 # recently enough to fully support this clang version.
-OUR_LLVM_REVISION=llvmorg-18-init-4631-gd50b56d1
+OUR_LLVM_REVISION=llvmorg-18.1.8
 
 mkdir $SRC/chromium_tools
 cd $SRC/chromium_tools
@@ -116,6 +116,7 @@ cmake -G "Ninja" \
  -DLLVM_TARGETS_TO_BUILD="$TARGET_TO_BUILD" \
  -DLLVM_ENABLE_PROJECTS="clang;lld" \
  -DLLVM_BINUTILS_INCDIR="/usr/include/" \
+ -DLIBCXXABI_USE_LLVM_UNWINDER=OFF \
  $LLVM_SRC/llvm
 
 ninja -j $NPROC
@@ -202,6 +203,7 @@ function cmake_libcxx {
  -DLIBCXX_ENABLE_SHARED=OFF \
  -DLIBCXX_ENABLE_STATIC_ABI_LIBRARY=ON \
  -DLIBCXXABI_ENABLE_SHARED=OFF \
+ -DLIBCXXABI_USE_LLVM_UNWINDER=OFF \
  -DCMAKE_BUILD_TYPE=Release \
  -DLLVM_ENABLE_PIC=ON \
  -DLLVM_TARGETS_TO_BUILD="$TARGET_TO_BUILD" \