[clangd] Fix detecting atomics in stand-alone builds

Include CheckAtomic CMake module from LLVM in order to detect support
for atomics when building stand-alone.  Otherwise,
the HAVE_CXX_ATOMICS64_WITHOUT_LIB variable is undefined and clangd
wrongly attempts to link -latomic on systems not using the library.

Original bug report: https://bugs.gentoo.org/667016

Differential Revision: https://reviews.llvm.org/D56061

llvm-svn: 350329