[cmake] Provide CURRENT_TOOLS_DIR centrally, replacing CLANG_TOOLS_DIR

CLANG_TOOLS_DIR holds the the current bin/ directory, maybe with a %(build_mode)
placeholder. It is used to add the just-built binaries to $PATH for lit tests.
In most cases it equals LLVM_TOOLS_DIR, which is used for the same purpose.
But for a standalone build of clang, CLANG_TOOLS_DIR points at the build tree
and LLVM_TOOLS_DIR points at the provided LLVM binaries.

Currently CLANG_TOOLS_DIR is set in clang/test/, clang-tools-extra/test/, and
other things always built with clang. This is a few cryptic lines of CMake in
each place. Meanwhile LLVM_TOOLS_DIR is provided by configure_site_lit_cfg().

This patch moves CLANG_TOOLS_DIR to configure_site_lit_cfg() and renames it:
 - there's nothing clang-specific about the value
 - it will also replace LLD_TOOLS_DIR, LLDB_TOOLS_DIR etc (not in this patch)

It also defines CURRENT_LIBS_DIR. While I removed the last usage of
CLANG_LIBS_DIR in e4cab4e2, there are LLD_LIBS_DIR usages etc that
may be live, and I'd like to mechanically update them in a followup patch.

Differential Revision: https://reviews.llvm.org/D121763