[clang/CMake] Respect LLVM_TOOLS_INSTALL_DIR

Otherwise clang installs all of its tools into `bin/` while
LLVM installs its tools into (LLVM_TOOLS_INSTALL_DIR).
I could swear this used to work (and in fact the julia build system
assumes it), but I can't pin down a specific commit that would
have broken this, and julia has been relying on pre-compiled binaries
for a while now (that don't use this setting), so it may have been
broken for quite a while.

Differential Revision: https://reviews.llvm.org/D88630

clang/cmake/modules/AddClang.cmake

@@ -159,7 +159,7 @@ macro(add_clang_tool name)
     get_target_export_arg(${name} Clang export_to_clangtargets)
     install(TARGETS ${name}
       ${export_to_clangtargets}
-      RUNTIME DESTINATION bin
+      RUNTIME DESTINATION ${LLVM_TOOLS_INSTALL_DIR}
       COMPONENT ${name})
 
     if(NOT LLVM_ENABLE_IDE)