Remove the "bool" return from OptionValue::Clear and its subclasses.

Remove the "bool" return from OptionValue::Clear and its subclasses.

Every override returns true and its return value is never checked.  I can't
see how clearing an OptionValue could fail, or what you would
do if it did.  The return serves no purpose.

Differential Revision: https://reviews.llvm.org/D84253