Use PlistSupport in a few more places

Switch over LogDiagnosticPrinter and introduce an integer helper.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212384 91177308-0d34-0410-b5e6-96231b3b80d8
writeARCDiagsToPlist(): make sure all diag levels get handled

Handle 'remark' level diagnostics as warnings instead of supplemental notes,
and use a covered switch to ensure the mapping is kept up to date.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212385 91177308-0d34-0410-b5e6-96231b3b80d8