Move DiagnosticReporter to errors.d

[jacob-carlborg]

It's more suitable located in errors.d since it's part of the error handling. It can also be used in other places then the lexer in the future.

Part of #9480.

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[thewilsonator]

Semaphore fails with

Performing "debug" build using /home/runner/dlang/dmd-2.085.0/linux/bin64/dmd for x86_64.
dmd:root ~master: target for configuration "library" is up to date.
dmd:lexer ~master: building configuration "library"...
dmd:parser ~master: building configuration "library"...
dmd:frontend ~master: building configuration "library"...
lexer ~master: building configuration "application"...
lexer.d(21,32): Error: undefined identifier StderrDiagnosticReporter
/home/runner/dlang/dmd-2.085.0/linux/bin64/dmd failed with exit code 1.

[jacob-carlborg]

Fixed, hopefully.

It's quite annoying that there seems to be no easy way to run all the test. I'm guessing it's because different tests are run on different CI platforms.

[jacob-carlborg]

I don't see why Semaphore is failing.

[WalterBright]

class DiagnosticReporter looks like hundreds of lines of code that does nothing. Why is this even there?

[WalterBright]

Apparently it was added by #9350