Priori to this patch, StrBoolAttr attributes had the strange property to be either unset, set to false or set to true. And the associated value was a string containing either "true" or "false". This is both cumbersome to use and inefficient.
This patch propose to no longer use the value filed, and just check the presence of the attribute, which was already an established practice in part of the LLVM codebase, see for instance https://reviews.llvm.org/D96400
As a side effect, when compiling sqlite3.c source code in -O0, this change makes the instruction count decrease from 6,157,211,048 to 6,155,407,798. Simpler and faster.