Comment parsing: Don't recognize commands in single-line double quotation

This is consistent with the behavior of Doxygen, and allows users to
write strings with C escapes or document input/output formats containing
special characters (@ or \) without escaping them, which might be
confusing. For example, if a function wants to document its expected
input format as "user@host" it doesn't have to write user\@host instead,
which would look right in the documentation but confusing in the code.
Now users can just use double quotes (which they might do anyway).

This fixes a lot of false positives of -Wdocumentation-unknown-command,
but it could also fix issues with -Wdocumentation if the text triggers
an actual command.

Reviewed By: gribozavr2

Differential Revision: https://reviews.llvm.org/D116190