[libcxx] [cmake] Allow testing against installed LLVM with no sources

[mgorny]

Summary

Adjust the stand-alone build files to accept either CMake files from LLVM_CMAKE_PATH or from LLVM_MAIN_SRC_DIR instead of requiring both. This makes it possible to run libcxx tests on top of installed LLVM and lit, without having to unpack a copy of LLVM sources. Furthermore, it avoids adding duplicate paths.

[mgorny]

Combined with D25076 (on llvm), this lets me run libcxx test suite on top of installed LLVM+lit, without LLVM sources.

[mgorny]

Gentle ping.

[beanz]

LGTM!

[mgorny]

Thanks for the review. Committed now.

cmake/Modules/HandleOutOfTreeLLVM.cmake

elseif(LLVM_CONFIG_PATH)
     set(LLVM_BINARY_DIR ${LLVM_OBJ_ROOT} CACHE PATH "Path to LLVM build tree")
     set(LLVM_MAIN_SRC_DIR ${MAIN_SRC_DIR} CACHE PATH "Path to LLVM source tree")
     set(LLVM_CMAKE_PATH "${LLVM_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm")
   else()
     set(LLVM_FOUND OFF)
     return()
   endif()
 
-  if (NOT EXISTS ${LLVM_MAIN_SRC_DIR})
-    set(LLVM_FOUND OFF)
-    message(WARNING "Not found: ${LLVM_MAIN_SRC_DIR}")
-    return()
-  endif()
-
-  if(NOT EXISTS ${LLVM_CMAKE_PATH})
+  if (EXISTS "${LLVM_CMAKE_PATH}")
+    list(APPEND CMAKE_MODULE_PATH "${LLVM_CMAKE_PATH}")
+  elseif (EXISTS "${LLVM_MAIN_SRC_DIR}/cmake/modules")
+    list(APPEND CMAKE_MODULE_PATH "${LLVM_MAIN_SRC_DIR}/cmake/modules")
+  else()
     set(LLVM_FOUND OFF)
-    message(WARNING "Not found: ${LLVM_CMAKE_PATH}")
+    message(WARNING "Neither ${LLVM_CMAKE_PATH} nor ${LLVM_MAIN_SRC_DIR}/cmake/modules found")
     return()
   endif()
 
-  list(APPEND CMAKE_MODULE_PATH "${LLVM_CMAKE_PATH}")
-  list(APPEND CMAKE_MODULE_PATH "${LLVM_MAIN_SRC_DIR}/cmake/modules")
-
   set(LLVM_FOUND ON)
 endmacro(find_llvm_parts)
 
 # If this is a standalone build not running as an external project of LLVM
 # we need to later make some decisions differently.
 if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   # The intent is that this doesn't necessarily mean the LLVM is installed (it
   # could be a build directory), but it means we need to treat the LLVM