Cannot build native clang-tidy-confusable-chars-gen when cross-compiling standalone clang #160530
Description
In the situation where you're cross-compiling clang standalone (that is, from inside clang/ against an existing llvm) the build doesn't know that it needs to build clang-tidy-confusable-chars-gen natively and instead builds it for the target and then fails when it tried to run the target binary.
I chased that down to LLVM_USE_HOST_TOOLS not actually being set outside of the top-level llvm/CMakeLists.txt, so a standalone clang build doesn't actually have that set.
If I then manually set that, it fails to find get_native_tool_path, which is easily solved by adding include(CrossCompile) to the standalone case in clang/CMakeLists.txt.
But now it's failing with:
ninja: error: 'tools/extra/clang-tidy/misc/CONFIGURE_Clang_NATIVE', needed by '/bin/clang-tidy-confusable-chars-gen', missing and no known rule to make it
This I can't seem to figure out. A top-level cross-compile of clang certainly does do the right thing though, so this should be possible to make work.