[CMake] Allow LLVM_EXTERNAL_<proj>_SOURCE_DIR to be overridden if it is empty.

[ychen]

Summary

This makes adding projects to LLVM_ENABLE_PROJECTS possible. Also its type should be PATH.

https://bugs.llvm.org/show_bug.cgi?id=42698

[ychen]

https://github.com/llvm/llvm-project/blob/master/llvm/docs/CMake.rst and some other places in the codebase refer to the type as PATH.

[beanz]

Adding ‘FORCE’ makes this not user overridable, which is the whole point of this being a cache variable.

[ychen]

• Override LLVM_EXTERNAL_<proj>_SOURCE_DIR when it is empty.

[ychen]

In D65045#1594693, @beanz wrote:

Adding ‘FORCE’ makes this not user overridable, which is the whole point of this being a cache variable.

Thanks. Add the FORCE flag only when the existing variable is empty.

[ychen]

• update

[ychen]

ping:)

[beanz]

LGTM. At some point (after the mono-repo becomes the source of truth) we need to clean all this up.

llvm/trunk/CMakeLists.txt

foreach(proj ${LLVM_ALL_PROJECTS} ${LLVM_EXTERNAL_PROJECTS})
     string(REGEX REPLACE "-" "_" upper_proj ${upper_proj})
     if ("${proj}" IN_LIST LLVM_ENABLE_PROJECTS)
       message(STATUS "${proj} project is enabled")
       set(SHOULD_ENABLE_PROJECT TRUE)
       set(PROJ_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../${proj}")
       if(NOT EXISTS "${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}" CACHE STRING "")
+      if( LLVM_EXTERNAL_${upper_proj}_SOURCE_DIR STREQUAL "" )
+        set(LLVM_EXTERNAL_${upper_proj}_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../${proj}" CACHE PATH "" FORCE)
+      else()
+        set(LLVM_EXTERNAL_${upper_proj}_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../${proj}" CACHE PATH "")
+      endif()
     elseif ("${proj}" IN_LIST LLVM_EXTERNAL_PROJECTS)
       message(STATUS "${proj} project is enabled")
       set(SHOULD_ENABLE_PROJECT TRUE)
     else()
       message(STATUS "${proj} project is disabled")
       set(SHOULD_ENABLE_PROJECT FALSE)
     endif()
     # Force `LLVM_TOOL_${upper_proj}_BUILD` variables to have values that