When performing cross in-tree builds, we need native versions of various
tools, we cannot assume the cross builds that are part of the current
build are executable. LLVM provides the setup_host_tool function to
handle this, either picking up versions of tools from
LLVM_NATIVE_TOOL_DIR, or implicitly building native versions as needed.
Use it for libclc too.
LLVM's setup_host_tool function assumes the project is LLVM, so this
also needs libclc's project() to be conditional on it being built
standalone. Luckily, the only change this needs is using
CMAKE_CURRENT_SOURCE_DIR instead of PROJECT_SOURCE_DIR.
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