[clangd] Add std::size_t to StdSymbol mapping

This is a common symbol that's missing from our mapping because
cppreference yields multiple headers.

Add it manually by picking cstddef to prevent insertion of some stdlib-internal
headers instead.

Fixes https://github.com/clangd/clangd/issues/666.

Differential Revision: https://reviews.llvm.org/D95423