Facilitate pre-installed Clang detection for in-tree builds

Comply with LLORG community changes in llvm/llvm-project@a1e6565.

Signed-off-by: Artem Gindinson <artem.gindinson@intel.com>

Original commit:
KhronosGroup/SPIRV-LLVM-Translator@efa761b

Author: Artem Gindinson

llvm-spirv/.github/workflows/check-in-tree-build.yml

@@ -54,12 +54,11 @@ jobs:
           echo "deb https://apt.llvm.org/bionic/ llvm-toolchain-bionic main" | sudo tee -a /etc/apt/sources.list
           echo "deb https://packages.lunarg.com/vulkan bionic main" | sudo tee -a /etc/apt/sources.list
           sudo apt-get update
-          # Linux systems in GitHub Actions already have a pre-installed clang
-          # that is not needed for the translator as we rely on newer version
-          sudo apt-get --purge remove clang-8 clang-9
           sudo apt-get -yq --no-install-suggests --no-install-recommends install \
             clang-${{ env.LLVM_VERSION }} \
             spirv-tools
+          # Linux systems in GitHub Actions already have older versions of clang
+          # pre-installed. Make sure to override these with the relevant version.
           sudo update-alternatives --install /usr/bin/clang clang /usr/bin/clang-${{ env.LLVM_VERSION }} 1000
       - name: Checkout LLVM sources
         uses: actions/checkout@v2

llvm-spirv/.github/workflows/check-out-of-tree-build.yml

@@ -47,15 +47,14 @@ jobs:
           echo "deb https://apt.llvm.org/bionic/ llvm-toolchain-bionic main" | sudo tee -a /etc/apt/sources.list
           echo "deb https://packages.lunarg.com/vulkan bionic main" | sudo tee -a /etc/apt/sources.list
           sudo apt-get update
-          # Linux systems in GitHub Actions already have a pre-installed clang
-          # that is not needed for the translator as we rely on newer version
-          sudo apt-get --purge remove clang-8 clang-9
           sudo apt-get -yq --no-install-suggests --no-install-recommends install \
             clang-${{ env.LLVM_VERSION }} \
             llvm-${{ env.LLVM_VERSION }}-dev \
             libomp-${{ env.LLVM_VERSION }}-dev \
             llvm-${{ env.LLVM_VERSION }}-tools \
             spirv-tools
+          # Linux systems in GitHub Actions already have older versions of clang
+          # pre-installed. Make sure to override these with the relevant version.
           sudo update-alternatives --install /usr/bin/clang clang /usr/bin/clang-${{ env.LLVM_VERSION }} 1000
       - name:  Checkout the translator sources
         uses: actions/checkout@v2

llvm-spirv/test/lit.cfg.py

@@ -41,7 +41,9 @@
 
 llvm_config.use_default_substitutions()
 
-llvm_config.use_clang()
+# Explicitly set `use_installed` to alleviate downstream CI pipelines of
+# any additional environment setup for pre-installed Clang usage.
+llvm_config.use_clang(use_installed=True)
 
 config.substitutions.append(('%PATH%', config.environment['PATH']))