[clang] Fix incorrect call to TextDiagnostic::printDiagnosticMessage

As per the documentation, the 2nd argument in printDiagnosticMessage
should be a bool that specifies whether the underlying message is a
continuation note diagnostic or not. More specifically, it should be:
```
Level == DiagnosticsEngine::Note
```
instead of:
```
Level
```

This change means that `no input file` in the following scenario will be
now correctly printed in bold:
```
$ bin/clang
clang: error: no input files
```
In terminals that don't support text formatting the behaviour doesn't
change.

Differential Revision: https://reviews.llvm.org/D87816