-XepAllSuggestionsAsWarnings doesn't work for checks manually enabled with default severity

[Marcono1234]

Version

Error Prone 2.23.0
JDK 17

Description

The -XepAllSuggestionsAsWarnings flag added by #3301 does not seem to work properly when you manually enable a check which is disabled by default, without overwriting its default severity.

When the default severity is 'suggestion', instead of being increased to 'warning' it remains at 'suggestion'.

I assume the underlying reason is:

• The severity is increased here
  

  error-prone/check_api/src/main/java/com/google/errorprone/scanner/ScannerSupplier.java

  Lines 172 to 176 in 7acf133

  	if (errorProneOptions.isSuggestionsAsWarnings()) {
  	getAllChecks().values().stream()
  	.filter(c -> c.defaultSeverity() == SeverityLevel.SUGGESTION)
  	.forEach(c -> severities.put(c.canonicalName(), SeverityLevel.WARNING));
  	}

• But then here it is overwritten again with the default
  

  error-prone/check_api/src/main/java/com/google/errorprone/scanner/ScannerSupplier.java

  Lines 213 to 215 in 7acf133

  	case DEFAULT:
  	severities.put(check.canonicalName(), check.defaultSeverity());
  	disabled.remove(check.canonicalName());

Maybe that second code snippet could use severities.putIfAbsent instead (haven't test it yet though)?

Reproduction steps

• Use -XepAllSuggestionsAsWarnings
• Use -Xep:SwitchDefault to enable that check, because it is disabled by default
  (SwitchDefault has the default severity 'suggestion')

Bug: SwitchDefault is reported as 'suggestion' instead of 'warning'

Workaround

When enabling the check also explicitly specify the severity, e.g. -Xep:SwitchDefault:WARN, but that has these disadvantages

• you might forget to do this (in case you don't apply :WARN for all checks), or it might be missing after a newer Error Prone version reduced the default severity of a check to 'suggestion'
• you might accidentally reduce the severity if you apply :WARN for all manually enabled checks, but some of them had 'error' as default severity