[clangd] Resolve driver symlinks, and look up unknown relative drivers in PATH.

Summary:
This fixes a reported bug: if clang and libc++ are installed under
/usr/lib/llvm-11/...  but there'- a symlink /usr/bin/clang++-11, then a
compile_commands.json with "/usr/bin/clang++-11 -stdlib=libc++" would previously
look for libc++ under /usr/include instead of /usr/lib/llvm-11/include.
The PATH change makes this work if the compiler is just "clang++-11" too.

As this is now doing IO potentially on every getCompileCommand(), we cache
the results for each distinct driver.

While here:
- Added a Memoize helper for this as multithreaded caching is a bit noisy.
- Used this helper to simplify QueryDriverDatabase and reduce blocking there.
  (This makes use of the fact that llvm::Regex is now threadsafe)

Reviewers: kadircet

Subscribers: jyknight, ormris, ilya-biryukov, MaskRay, jkorous, arphaman, jfb, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75414