[Comgr] Allow users to pass LLVM_LIT_PATH to CMake

Some builds/installations have a directory setup such that
we cannot deduce the path to LLVM build directory from the Comgr
build directory. In these cases, a user can manually pass the path
to the LLVM_LIT script to CMake via -DLLVM_LIT_PATH

Change-Id: I9a31ef401602469a7efb6a316807e5f5c0cdb743

Author: lamb-j

amd/comgr/test-lit/CMakeLists.txt

@@ -10,12 +10,14 @@ cannonicalize_cmake_boolean(COMGR_DISABLE_SPIRV)
 
 configure_file(lit.site.cfg.py.in lit.site.cfg.py @ONLY)
 
-# Comgr source build
-if (EXISTS "${LLVM_TOOLS_BINARY_DIR}/../../bin/llvm-lit")
-  set(LLVM_LIT_PATH "${LLVM_TOOLS_BINARY_DIR}/../../bin/llvm-lit")
-# LLVM external projects build
-else()
-  set(LLVM_LIT_PATH "${LLVM_TOOLS_BINARY_DIR}/llvm-lit")
+if (NOT DEFINED LLVM_LIT_PATH)
+  # Comgr source build
+  if (EXISTS "${LLVM_TOOLS_BINARY_DIR}/../../bin/llvm-lit")
+    set(LLVM_LIT_PATH "${LLVM_TOOLS_BINARY_DIR}/../../bin/llvm-lit")
+  # LLVM external projects build
+  else()
+    set(LLVM_LIT_PATH "${LLVM_TOOLS_BINARY_DIR}/llvm-lit")
+  endif()
 endif()
 
 add_custom_target(test-lit COMMAND "${LLVM_LIT_PATH}"