[nfc] Fix leak after creating `ItaniumMangleContext`.
[nfc] Fix leak after creating `ItaniumMangleContext`.

We need to delete the `MangleContext` after we create it. This follows the existing use of `ItaniumMangleContext::create` in ASTMangler.cpp and a similar change in llvm here: llvm/llvm-project/commit/8b5783194ced98cabaa585678cacaf7c2e2763d8

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