TextDiagnostic: print remark level diagnostics in bold too

The purpose of bolding these is to make them visually distinct from
continuations (supplemental note diagnostics). Therefore, the bolding applies
to all severities _including_ remarks -- it's not in any way an indicator of
priority. Also simplify and comment.

No tests.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211447 91177308-0d34-0410-b5e6-96231b3b80d8
DiagnosticRenderer: emit basic notes as real diagnostics

Fixes terminal column wrapping and vestigial 'note:' prefixes that would appear
when using emitBasicNote().

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211448 91177308-0d34-0410-b5e6-96231b3b80d8