[clangd] Have visibleNamespaces() and getEligiblePoints() take a LangOptions...

[clangd] Have visibleNamespaces() and getEligiblePoints() take a LangOptions rather than a FormatStyle

Summary:
These functions only use the FormatStyle to obtain a LangOptions via
format::getFormattingLangOpts(), and some callers can more easily obtain
a LangOptions more directly.

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75716