Fix MA0075 false positive on Enum.ToString and handle conditional access

[meziantou]

Fixes #1310

MA0075 vs MA0185 on enums

MA0075 reported Do not use implicit culture-sensitive ToString for "abc" + enumValue.ToString("G"), while MA0185 considered the very same value culture invariant.

The ToString overloads of an enum are declared on System.Enum, so invocation.TargetMethod.ContainingType is System.Enum. That type is a class, so IsEnum() returns false, and it implements IFormattable, so it was classified as culture sensitive. System.Enum ignores the format provider, so it is now part of the known culture-insensitive types.

Adding System.Enum to the type-level check rather than using invocation.Instance?.Type also covers values statically typed as System.Enum and type parameters constrained to where T : struct, Enum.

Note that the underlying gap is not new: it only became visible in #1273, which changed the string concatenation rule from analyzing only operands implicitly converted to object to analyzing every operand. Before that, a string-typed operand such as enumValue.ToString("G") was never analyzed at all.

Conditional access

Found while investigating the above: IConditionalAccessOperation was not handled, so it fell through to the generic branch that uses the type of the whole expression. For "abc" + dateTime?.ToString("F") that type is string, so the rule never reported it. The culture sensitivity is now computed from WhenNotNull.

This only affects MA0075/MA0076. MA0185 reaches interpolation holes through IInterpolatedStringAppendOperation, which works off argument types, so its behavior is unchanged.

Tests

• Enums: concat with every format (G g F f D d X x, default(string), a non-constant string format), plain ToString(), implicit concat, nullable enums, System.Enum-typed values, user-defined enums, where T : struct, Enum, and the interpolated-string forms.
• Conditional access: value?.ToString(), value?.ToString("F"), value?.ToString(format), value?.Date.ToString("F"), value?.Ticks.ToString(), nested value?.Child?.Value.ToString("F"), value?.Value, and the interpolated form are reported; value?.ToString("o"), value?.ToString(CultureInfo.InvariantCulture), value?.Ticks.ToString("X") and value?.Kind.ToString("G") are not.
• MA0185 enum tests, so the two rules cannot drift apart again without a test failing.

"abc" + value?.Ticks.ToString() is reported on purpose: Ticks is a non-constant long, and MA0075 already treats non-constant signed integers as culture sensitive because of the negative sign.

Note for the reviewer

Enum.ToString("D") on an undefined value of a signed enum formats the underlying integer with the current culture, so a negative value picks up the culture's negative sign. Treating System.Enum as unconditionally invariant ignores that. This is consistent with what MA0075 already did for "abc" + enumValue and with MA0185, but it is not literally always invariant.

Validation

• dotnet test --max-parallel-test-modules 2: 18327 passed, 0 failed (all five Roslyn versions)
• dotnet run --project src/DocumentationGenerator: exit code 0, no generated markdown change