[Kernel CI] Use Fuchsia Clang

This is a fully tested nightly LLVM toolchain built by Google LUCI, "live at HEAD"

Introduction: ClangBuiltLinux/llvm-distributors-conf-2021#14
CI Status: https://luci-milo.appspot.com/ui/p/fuchsia/builders/luci.fuchsia.prod/clang-linux-x64
Repo: https://chrome-infra-packages.appspot.com/p/fuchsia/third_party/clang/linux-amd64

.github/workflows/build-lts.yml

@@ -42,7 +42,10 @@ jobs:
     - name: Install dependencies
       id: dep
       run: |
-        pacman -Syu --noconfirm pahole xmlto inetutils bc cpio jq llvm llvm-libs clang lld ccache
+        pacman -Syu --noconfirm pahole xmlto inetutils bc cpio jq ccache go
+        GOBIN=/usr/bin go install go.chromium.org/luci/cipd/client/cmd/...@latest
+        cipd install fuchsia/third_party/clang/linux-amd64 latest -root /usr/local/fuchsia-clang
+        echo "PATH=/usr/local/fuchsia-clang/bin:$PATH" >> $GITHUB_ENV
 
     - name: Trust this directory
       run: git config --global --add safe.directory '*' # v2.35.3 or later
@@ -58,7 +61,7 @@ jobs:
         scripts/config -e ${{ matrix.arch }}
 
         # Load version info into env
-        echo "CLANG_VERSION=$(pacman -Qs clang | grep local/clang | sed "s#.*local/clang \(.*\)#\1#")" | tee -a $GITHUB_ENV
+        echo "CLANG_VERSION=$(clang -dumpversion)" | tee -a $GITHUB_ENV
         
         export CURRENT_VERSION=$(make kernelrelease)
         # must query with a token, or will fail with api rate limit on public runners

.github/workflows/build.yml

@@ -42,7 +42,10 @@ jobs:
     - name: Install dependencies
       id: dep
       run: |
-        pacman -Syu --noconfirm pahole xmlto inetutils bc cpio jq llvm llvm-libs clang lld ccache
+        pacman -Syu --noconfirm pahole xmlto inetutils bc cpio jq ccache go
+        GOBIN=/usr/bin go install go.chromium.org/luci/cipd/client/cmd/...@latest
+        cipd install fuchsia/third_party/clang/linux-amd64 latest -root /usr/local/fuchsia-clang
+        echo "PATH=/usr/local/fuchsia-clang/bin:$PATH" >> $GITHUB_ENV
 
     - name: Trust this directory
       run: git config --global --add safe.directory '*' # v2.35.3 or later
@@ -64,7 +67,7 @@ jobs:
         scripts/config -e ${{ matrix.arch }}
 
         # Load version info into env
-        echo "CLANG_VERSION=$(pacman -Qs clang | grep local/clang | sed "s#.*local/clang \(.*\)#\1#")" | tee -a $GITHUB_ENV
+        echo "CLANG_VERSION=$(clang -dumpversion)" | tee -a $GITHUB_ENV
         
         export CURRENT_VERSION=$(make kernelrelease)
         # must query with a token, or will fail with api rate limit on public runners