[llvm] [libc][workflows] fix fullbuild by installing clang-20/clang++-20 (PR #136074)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-github-workflow

Author: Schrodinger ZHU Yifan (SchrodingerZhu)

<details>
<summary>Changes</summary>



---
Full diff: https://github.com/llvm/llvm-project/pull/136074.diff


1 Files Affected:

- (modified) .github/workflows/libc-fullbuild-tests.yml (+7-4) 


``````````diff
diff --git a/.github/workflows/libc-fullbuild-tests.yml b/.github/workflows/libc-fullbuild-tests.yml
index 00d2696e7e586..54d0df99b687e 100644
--- a/.github/workflows/libc-fullbuild-tests.yml
+++ b/.github/workflows/libc-fullbuild-tests.yml
@@ -19,13 +19,13 @@ jobs:
         include:
           - os: ubuntu-24.04
             ccache-variant: sccache
-            c_compiler: clang
-            cpp_compiler: clang++
+            c_compiler: clang-20
+            cpp_compiler: clang++-20
           # TODO: remove ccache logic when https://github.com/hendrikmuhs/ccache-action/issues/279 is resolved.
           - os: ubuntu-24.04-arm
             ccache-variant: ccache
-            c_compiler: clang
-            cpp_compiler: clang++
+            c_compiler: clang-20
+            cpp_compiler: clang++-20
           # TODO: add back gcc build when it is fixed
           # - c_compiler: gcc
           #   cpp_compiler: g++
@@ -51,6 +51,9 @@ jobs:
     #   For more information, see https://wiki.debian.org/Multiarch/LibraryPathOverview
     - name: Prepare dependencies (Ubuntu)
       run: |
+        wget https://apt.llvm.org/llvm.sh
+        chmod +x llvm.sh
+        sudo ./llvm.sh 20
         sudo apt-get update
         sudo apt-get install -y libmpfr-dev libgmp-dev libmpc-dev ninja-build linux-libc-dev
         sudo ln -sf /usr/include/$(uname -p)-linux-gnu/asm /usr/include/asm

``````````

</details>


https://github.com/llvm/llvm-project/pull/136074