[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

(cherry picked from commit 6104e14)
(cherry picked from commit f252e17)
(cherry picked from commit 9039ce8)

Author: Keno

clang/cmake/modules/AddClang.cmake

@@ -169,7 +169,7 @@ macro(add_clang_tool name)
       get_target_export_arg(${name} Clang export_to_clangtargets)
       install(TARGETS ${name}
         ${export_to_clangtargets}
-        RUNTIME DESTINATION "${CMAKE_INSTALL_BINDIR}"
+        RUNTIME DESTINATION "${LLVM_TOOLS_INSTALL_DIR}"
         COMPONENT ${name})
 
       if(NOT LLVM_ENABLE_IDE)