[CMake] Honor LLVM_EXTERNAL_<proj>_SOURCE_DIR

When LLVM_ENABLE_PROJECTS is set, CMake assumes the project
directories are all side-by-side. This is not always the case and
there's no reason to expect it if LLVM_EXTERNAL_<proj>_SOURCE_DIR is
set. Honor that setting if it exists and allow the build configuration
to continue.

Differential Revision: https://reviews.llvm.org/D49672

llvm-svn: 354693

llvm/CMakeLists.txt

@@ -140,7 +140,7 @@ if (LLVM_ENABLE_PROJECTS_USED OR NOT LLVM_ENABLE_PROJECTS STREQUAL "")
       if(NOT EXISTS "${PROJ_DIR}" OR NOT IS_DIRECTORY "${PROJ_DIR}")
         message(FATAL_ERROR "LLVM_ENABLE_PROJECTS requests ${proj} but directory not found: ${PROJ_DIR}")
       endif()
-      set(LLVM_EXTERNAL_${upper_proj}_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../${proj}")
+      set(LLVM_EXTERNAL_${upper_proj}_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../${proj}" CACHE STRING "")
     else()
       message(STATUS "${proj} project is disabled")
       set(SHOULD_ENABLE_PROJECT FALSE)