regex: make `ExperimentalRegexBridging.h` C friendly
regex: make `ExperimentalRegexBridging.h` C friendly

`bool` is not a valid type in C.  However, C99 does provide `stdboo.h`
to help bridge that type.

roambotics/swift,ahoppen/swift,benlangmuir/swift,benlangmuir/swift,apple/swift,roambotics/swift,rudkx/swift,apple/swift,glessard/swift,ahoppen/swift,gregomni/swift,glessard/swift,rudkx/swift,atrick/swift,gregomni/swift,JGiola/swift,JGiola/swift,ahoppen/swift,gregomni/swift,atrick/swift,gregomni/swift,benlangmuir/swift,atrick/swift,apple/swift,roambotics/swift,rudkx/swift,ahoppen/swift,roambotics/swift,glessard/swift,ahoppen/swift,benlangmuir/swift,atrick/swift,roambotics/swift,roambotics/swift,apple/swift,rudkx/swift,glessard/swift,apple/swift,gregomni/swift,glessard/swift,glessard/swift,JGiola/swift,atrick/swift,atrick/swift,ahoppen/swift,JGiola/swift,JGiola/swift,rudkx/swift,gregomni/swift,benlangmuir/swift,apple/swift,benlangmuir/swift,JGiola/swift,rudkx/swift