PlistSupport: Unify ARCMigrate / StaticAnalyzer plist writers

Reduces the ARCMT migrator plist writer down to a single function,
arcmt::writeARCDiagsToPlist() which shares supporting functions with the
analyzer plist writer.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200075 91177308-0d34-0410-b5e6-96231b3b80d8