Keep \t and \f escapes in UseTextBlocks to avoid incidental whitespace stripping

[timtebeek]

What's changed

UseTextBlocks decodes each concatenated string literal to its value before emitting text-block content, which turns escape sequences like \t and \f into literal tab/form-feed characters. When such a literal ends up on a content line's leading edge, javac cannot distinguish it from incidental leading whitespace and strips it during text-block whitespace processing (JLS 3.10.6) — silently changing the string's value (e.g. "\tb" compiles to "b").

This re-escapes tab and form-feed characters back to \t / \f escape sequences when generating text-block content, right after backslash escaping so the introduced backslashes aren't doubled. Indentation added later is unaffected.

Testing

Added keepTabEscapeToAvoidIncidentalWhitespaceStripping and keepFormFeedEscapeToAvoidIncidentalWhitespaceStripping, matching the reproduction in the issue. Full UseTextBlocksTest suite passes.

• Fixes UseTextBlocks unescapes \t into a literal tab, which incidental-whitespace stripping can silently corrupt at compile time #1158