BaseTools: Add ClangToolChain build plugin

Add a new edk2-pytools build plugin that automatically configures
CLANG_BIN for CLANGPDB and CLANGDWARF tool chains. The plugin
resolves the clang binary path by checking, in order:

1. CLANG_BIN environment variable
2. System PATH
3. Default LLVM install path (Windows)
4. VS-installed clang (Windows)

The plugin also reports clang and linker (lld-link/lld) versions
to the version aggregator for build logging.

Signed-off-by: Aaron Pop <aaronpop@microsoft.com>