[clangd] Increase stack size of the new threads on macOS

Summary: By default it's 512K, which is way to small for clang parser to run on. There is no way to do it via platform-independent API, so it's implemented via pthreads directly in clangd/Threading.cpp.

Fixes https://github.com/clangd/clangd/issues/273

Patch by Dmitry Kozhevnikov!

Reviewers: ilya-biryukov, sammccall, arphaman

Reviewed By: ilya-biryukov, sammccall, arphaman

Subscribers: dexonsmith, umanwizard, jfb, ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D50993