[GSoC] Refactor DiagnosticEngine to use YAML files

[HassanElDesouky]

In this PR, I modified the current DiagnosticEngine class to be able to parse diagnostic messages from YAML files in the future.

Summary of changes:

• Created a DiagnosticNode struct to hold the objects of the YAML files.
• Defined the YAML format for DiagnosticNode
• Created a CMakeLists.txt file to handle where the YAML diagnostic files will be in the toolchain.
• Refactored DiagnosticEngine::diagnosticStringFor to handle returning the diagnostics messages from YAML files and handle fall-backs if the diagnostic message isn't present in the current YAML file.

Next steps after this PR gets merged:

• Retrieve the diagnostic messages path from the compiler toolchain.
• Create a -locale compiler frontend flag for choosing the language.
• Create a -diagnostic-messages-path compiler frontend flag to provide a path for the YAML files (to use in testing).
• Write tests.

cc @xedin

[xedin]

Overall looks great, definitely heading in the right direction! I have left some comments inline which we need to tackle before merging.

[HassanElDesouky]

@xedin I renamed the diag_id to just id

[xedin]

Looks great, thank you!

[xedin]

@swift-ci please smoke test

[xedin]

@swift-ci please smoke test

[HassanElDesouky]

@xedin Tests passed! 🥳

[davezarzycki]

This pull request is using undefined behavior. Can we fix this any time soon?

lib/AST/DiagnosticEngine.cpp:1093:12: warning: address of stack memory associated with local variable 'localizedMessage' returned [-Wreturn-stack-address]
    return localizedMessage.c_str();
           ^~~~~~~~~~~~~~~~

[HassanElDesouky]

@davezarzycki I'll fix it in the upcoming PR if that's okay?

[compnerd]

This seems to have caused other issues. This breaks when building with lld due to multiple definitions of the data types. enum classes are not COMDATed it seems, and this cause multiple conflicting definitions. Could you please fix that soon?