[6.2] parse Doxygen commands if a previous directive is being truncated

[QuietMisdreavus]

• Explanation: Doxygen commands should not be parsed if they would appear in the middle of a block directive, but the current detection is too stringent. Update the detection to only block Doxygen commands if we're in the middle of block directive contents.
• Scope: Allows some Doxygen/HeaderDoc-commented input (where commands are stacked next to each other without intervening blank lines) to correctly parse the commands that are supported even if they are preceded by unsupported commands.
• Issues: rdar://147921223
• Original PRs: parse Doxygen commands if a previous block directive is being truncated #223
• Risk: Low. This is a behavior change, but the situations that it occurs in are relatively uncommon and will only improve in functionality.
• Testing: Automated tests have been added.
• Reviewers: @d-ronnqvist

[QuietMisdreavus]

@swift-ci Please test