Toolchain+CI: Link LLVM with LLD if available

According to most benchmarks, LLD is faster than GNU ld and the macOS
linker, so let's use it if possible in order to speed up the toolchain
build.

Meta/Azure/Setup.yml

@@ -10,7 +10,7 @@ steps:
         sudo add-apt-repository ppa:canonical-server/server-backports
         sudo add-apt-repository ppa:ubuntu-toolchain-r/test
         sudo apt-get update
-        sudo apt-get install ccache e2fsprogs gcc-11 g++-11 libstdc++-11-dev libmpfr-dev libmpc-dev ninja-build qemu-utils qemu-system-i386 unzip
+        sudo apt-get install ccache e2fsprogs gcc-11 g++-11 libstdc++-11-dev libmpfr-dev libmpc-dev ninja-build qemu-utils qemu-system-i386 unzip lld
       displayName: 'Install Dependencies'
 
   - ${{ if eq(parameters.os, 'Linux') }}:

Toolchain/BuildClang.sh

@@ -130,6 +130,18 @@ then
     exit 1
 fi
 
+link_lld=
+buildstep dependencies echo "Checking whether the LLD linker is available..."
+if ${CXX:-c++} -o /dev/null -fuse-ld=lld -xc - >/dev/null 2>/dev/null << 'PROGRAM'
+int main() {}
+PROGRAM
+then
+    link_lld=1
+    buildstep dependencies echo "Using LLD for linking LLVM."
+else
+    buildstep dependencies echo "LLD not found. Using the default linker."
+fi
+
 # === CHECK CACHE AND REUSE ===
 
 pushd "$DIR"
@@ -254,6 +266,7 @@ pushd "$DIR/Build/clang"
             -DCMAKE_INSTALL_PREFIX="$PREFIX" \
             -DSERENITY_MODULE_PATH="$DIR/CMake" \
             -C "$DIR/CMake/LLVMConfig.cmake" \
+            ${link_lld:+"-DLLVM_ENABLE_LLD=ON"} \
             ${dev:+"-DLLVM_CCACHE_BUILD=ON"} \
             ${ci:+"-DLLVM_CCACHE_BUILD=ON"} \
             ${ci:+"-DLLVM_CCACHE_DIR=$LLVM_CCACHE_DIR"} \