[libclc] [llvm] [libclc] Support LLVM_ENABLE_RUNTIMES when building (PR #141574)

[Joseph Huber via cfe-commits]

================
@@ -72,7 +72,7 @@ else()
   # Note that we check this later (for both build types) but we can provide a
   # more useful error message when built in-tree. We assume that LLVM tools are
   # always available so don't warn here.
-  if( NOT clang IN_LIST LLVM_ENABLE_PROJECTS )
+  if( NOT LLVM_RUNTIMES_BUILD AND NOT clang IN_LIST LLVM_ENABLE_PROJECTS )
----------------
jhuber6 wrote:

The runtimes builds pass this `USE_TOOLCHAIN` argument which tells the external project utility to grab the `clang` from the build directory. If we don't have clang it might be more reasonable to just emit a warning there. @petrhosek Is there a reason we don't already emit a warning / error if the user does a runtimes build without clang to bootstrap?

https://github.com/llvm/llvm-project/pull/141574