Fix bugzilla issue 24871 - DDoc strips indent in triple slash comments

[ArthaTi]

Removes code in the lexer responsible for removing leading spaces in triple slash doc comments.

This affects the test file for issue 14413; two more spaces appear, but they are not significant in the HTML output.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	24871	trivial	DDoc strips indent in triple slash comments

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#17082"

[ArthaTi]

There's a few CI failures. I do not understand the logs from Azure pipelines. The buildkite error is from exactly the same mismatch that happens in compiler/test/compilable/extra-files/ddoc14413.html, just in DDox's test suite. I don't know what to do about this.

[dkorpel]

Three options are:

• Send a PR at the source https://github.com/dlang/ddox
• Patch it in the buildkite script: https://github.com/dlang/ci/blob/master/buildkite.sh
• Edit this PR so the behavior for that DDox test stays the same

Fixing DDox is nontrivial because when you change the expected output, it will fail as long as it's using an older DMD, so you need to make the test pass with dmd before and after this PR. Removing the test in the buildkite script works as a quick hack, but it's not ideal.

Maybe you can still strip 1 space after ///? I think usually people write:

/// Lorem
/// Ipsum

Rather than:

///Lorem
///Ipsum