[SAoC 2021] Solve Dependency Hell: Milestone 1, Week 3 & 4

[Ahmet Sait]

[Milestone 
Overview](https://gist.github.com/ahmetsait/10925a246b0d412a698e927658f3f363#milestone-overview)

Hi,
These two weeks I've been mostly trying to get 
[mangle-prefix](https://github.com/dlang/dmd/pull/13115) branch 
into mergable state.

Things I've managed to do:
- Implemented mangling for ModuleInfo.
- Propagated dmd frontend changes to headers for C++ integration.
- Addressed some of the PR code reviews.
- Wrote tests for -mangle-prefix implementation.
- Made sure CI tests are passing. A significant amount of my time 
was spent debugging CI failures.

Meanwhile the PR waits in review queue forever I'll start working 
on implementing 
[-import-local](https://github.com/ahmetsait/dmd/tree/import-local) switch. This new switch works similar to the regular import switch (`-I`) except it will allow imports to only affect specific source files. The `-mangle-prefix` (which eliminates link-time symbol collision) combined with `-import-local` (which eliminates compile-time name lookup collision) I should be able to solve a minimal [dependency hell](https://github.com/ahmetsait/dub-dep-hell) test case.