Add a cmake warning when someone tries to configure clang-tools-extra without clang

[njames93]

Summary

clang-tools-extra targets wont be generated without clang being enabled, so to help users know why their build commands are failing emit a warning from cmake.
Still unsure if the correct behaviour would be to do what flang does and automatically enable its dependency - mlir.

[njames93]

Fix incorrect quote location

[njames93]

Ping

[stephenneuendorffer]

I'd advocate for issuing a message (along the lines of "clang-tools-extra is enabled, which depends on 'clang'. Automatically enabling 'clang'." and 'doing the right thing' by enabling clang.
In fact, this seems to be a common enough paradigm that it should be handled in the cmake infrastructure.

[njames93]

Automatically enable clang as well as displaying warning

[DavidTruby]

I agree that we might want to abstract this somewhere as I can imagine other subprojects wanting to do it as well. It would probably be nicer for the dependency to appear in the subproject CMakeLists but I'm not sure if that's possible.
Perhaps we should change the STATUS message to a WARNING for flang too, or at least keep both the same?

[njames93]

Change message from WARNING to STATUS in line with flang.

llvm/CMakeLists.txt

 set(LLVM_EXTRA_PROJECTS "flang")
 # List of all known projects in the mono repo
 set(LLVM_KNOWN_PROJECTS "${LLVM_ALL_PROJECTS};${LLVM_EXTRA_PROJECTS}")
 set(LLVM_ENABLE_PROJECTS "" CACHE STRING
   "Semicolon-separated list of projects to build (${LLVM_KNOWN_PROJECTS}), or \"all\".")
 if( LLVM_ENABLE_PROJECTS STREQUAL "all" )
   set( LLVM_ENABLE_PROJECTS ${LLVM_ALL_PROJECTS})
 endif()
+if ("clang-tools-extra" IN_LIST LLVM_ENABLE_PROJECTS AND NOT "clang" IN_LIST LLVM_ENABLE_PROJECTS)
+  message(STATUS "clang-tools-extra is enabled, which depends on 'clang'. Automatically enabling 'clang'.")
+  list(APPEND LLVM_ENABLE_PROJECTS "clang")
+endif()
 if ("flang" IN_LIST LLVM_ENABLE_PROJECTS AND NOT "mlir" IN_LIST LLVM_ENABLE_PROJECTS)
   message(STATUS "Enabling MLIR as a dependency to flang")
   list(APPEND LLVM_ENABLE_PROJECTS "mlir")
 endif()
 
 # LLVM_ENABLE_PROJECTS_USED is `ON` if the user has ever used the
 # `LLVM_ENABLE_PROJECTS` CMake cache variable.  This exists for
 # several reasons: