[Stackmaps] Replace the custom ConstantPool class with a MapVector.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207803 91177308-0d34-0410-b5e6-96231b3b80d8
[Stackmaps] Refactor serialization code. No functional change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207804 91177308-0d34-0410-b5e6-96231b3b80d8
[Stackmaps] Add command line option to specify the stackmap version.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207805 91177308-0d34-0410-b5e6-96231b3b80d8