Why doesn't clangd LSP find my compiler's system headers?
When using clangd as LSP for C/C++ projects, e.g. in nvim, it
enables all the nice convenience features we're used to from
heavyweight IDEs like VS Code, Eclipse etc.
Jump to definition, jump to references, autocompletion etc.
But sometimes, when using more complex compiler setups like multiple different
compilers installed in parallel, cross compilation etc., it can suddenly fail
to find standard includes like <fstream>, <string> (aka system headers),
making it completely useless. What's happening there?