Switch Header/Source often chooses the wrong file

Since the change to use clangd to handle Switch Header/Source, behavior has been much less reliable compared to the built-in code model when header and source files are kept in separate directories. See upstream bug report here: https://github.com/clangd/clangd/issues/1617 Disabling clangd restores header/source swapping in these situations, but also sacrifices all the other related editor features.

One suggestion is to provide the ability to force the internal code model for header/source switching. This would allow affected users to use the old behavior until the clangd bugs are resolved.