Teach serialized diagnostics about notes without locations.

Along the way, improve a diagnostic for "previous declaration here" for implicit parameters.

Fixes <rdar://problem/13211384>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175802 91177308-0d34-0410-b5e6-96231b3b80d8
Remove accidentally introduced no-op line.

Was used during experiments, but another if-statements a few lines
before makes it (intentionally) useless.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175803 91177308-0d34-0410-b5e6-96231b3b80d8