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

[Fraser Cormack via cfe-commits]

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@@ -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 )
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frasercrmck wrote:

I'd quite like to keep this check/error if possible. The error you get if you forget `-LLVM_ENABLE_PROJECTS=clang` is horrible. Do you think it would it acceptable to pass through `LLVM_ENABLE_PROJECTS` to the runtimes build, @jhuber6?

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