[CMake] Allow setting the location of host tools with LLVM_NATIVE_TOOL_DIR

This avoids having to specify the location of all individual tools.
In current builds, one may want to specify LLVM_TABLEGEN, CLANG_TABLEGEN,
LLDB_TABLEGEN, LLVM_CONFIG_PATH, CLANG_PSEUDO_GEN and
CLANG_TIDY_CONFUSABLE_CHARS_GEN; specifying just the base directory
containing all of them is much more convenient.

Factorize the code for setting up use of a tool that is used during
the build (which either is newly built in the same build, or
built in a separate nested cmake build - when cross compiling or
when e.g. optimized tablegen is requested - or used from an existing
prebuilt binary).

Differential Revision: https://reviews.llvm.org/D131052