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

Author: SchrodingerZhu

.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