sccache llvm build

Author: lukel97

.github/workflows/test-suite/llvm.cmake

@@ -2,3 +2,5 @@ set(CMAKE_BUILD_TYPE Release CACHE STRING "")
 set(LLVM_TARGETS_TO_BUILD "AArch64;RISCV;X86" CACHE STRING "")
 set(LLVM_ENABLE_PROJECTS "clang;lld" CACHE STRING "")
 set(LLVM_APPEND_VC_REV OFF CACHE BOOL "")
+set(CMAKE_C_COMPILER_LAUNCHER sccache CACHE STRING "")
+set(CMAKE_CXX_COMPILER_LAUNCHER sccache CACHE STRING "")