[Issue 20134] autodecode should use replacementDchar rather than throwing on invalid

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20134

--- Comment #1 from Walter Bright <bugzilla at digitalmars.com> ---
Over time, common practice has evolved from rejecting malformed UTF to
replacing it with replacementDchar, which enables the application (like a web
browser) to continue processing.

Code should also be faster with this change.

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