devel/llvm*: Avoid extraneous escapes

All of these single quotes are safe on their own, as they're enclosed in
double quotes. Removes the escapes, because they're currently passed through
to the underlying regex engine as \' to be interpreted as '.

A future change will forbid this escape, then an even further future change
will actually imbue \' with special meaning based on GNU interpretation
("end of subject").

No PORTREVISION bump or rebuild is needed, as they're functionally
equivalent.

PR: 247058
Approved by: koobs (mentor), brooks (maintainer)
MFH: 2020Q2 (blanket: trivial build fix)