[X86] Simplify StackMapShadowTracker; NFC

 - Elide trivial contructor and desctructor
 - Move implementation out of an unnecessary explicit llvm namespace
   scope

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@266794 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed