[libclc] Support LLVM_ENABLE_RUNTIMES when building (#141574)

This commit deprecates the use of LLVM_ENABLE_PROJECTS in favour of
LLVM_ENABLE_RUNTIMES when building libclc.

Alternatively, using -DLLVM_RUNTIME_TARGETS=<triple> combined with
-DRUNTIMES_<triple>_LLVM_ENABLE_RUNTIMES=libclc also gets pretty far but
fails due to zlib problems building the LLVM utility 'prepare_builtins'.
I'm not sure what's going on there but I don't think it's required at
this stage. More work would be required to support that option.

This does nothing to change how the host tools are found in order to be
used to actually build the libclc libraries.

Note that under such a configuration the final libclc builtin libraries
are placed in `<build>/runtimes/runtimes-bins/libclc/`, which differs
from a non-runtimes build. The installation location remains the same.

Fixes #124013.

libclc/CMakeLists.txt

@@ -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 )
     message(FATAL_ERROR "Clang is not enabled, but is required to build libclc in-tree")
   endif()