TextDiagnosticPrinter: use the mapped level for remark flag computation

Custom diagnostics don't have a builtin class so this wouldn't have worked.
Reduces surface area of remark-related changes.

No test coverage.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@211462 91177308-0d34-0410-b5e6-96231b3b80d8