Demangling: degrade better with VS2017
Demangling: degrade better with VS2017

`_Pragma` support was added to VS with VS2019 U6, version 16.6, toolset
14.26.  Add a conditional to avoid the use of `_Pragma` prior to that
version.

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