[clangd] Add option to use dirty file contents when building preambles.

Adds a option `use-dirty-preambles` to enable using unsaved in editor contents when building pre-ambles.
This enables a more seamless user experience when switching between header and implementation files and forgetting to save inbetween.
It's also in line with the LSP spec that states open files in the editor should be used instead of on the contents on disk - https://microsoft.github.io/language-server-protocol/overviews/lsp/overview/
For now the option is defaulted to off and hidden, Though I have a feeling it should be moved into the `.clangd` config and possibly defaulted to true.

Addresses https://github.com/clangd/clangd/issues/488

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D95046