Fix Issue 9621 - std.conv.parseEscape fails on octals and named

[ghost]

This is an alternative to PR #7273. Meanwhile I learned, that octal literals have been removed for integers. Maybe it's better to remove them for string literals too and keep them away from the parse function?

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	9621	normal	std.conv.parseEscape fails on octals and named

Testing this PR locally

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

dub fetch digger
dub run digger -- build "master + phobos#7274"