gdb: cast return value of std::unique_ptr::release to void

gdb: cast return value of std::unique_ptr::release to void

My editor shows warnings like:

     value.c:2784: warning: The value returned by this function should be used
     value.c:2784: note: cast the expression to void to silence this warning [bugprone-unused-return-value]

These warnings come from clangd, so ultimately from one of the clang
static analyzers (probably clang-tidy).

Silence these warnings by casting to void.  Add a comment to explain
why this unusual thing is done.

Change-Id: I58323959c0baf9f1b20a8d596e4c58dc77c6809a
Approved-By: Tom Tromey <tom@tromey.com>