Fix MA0075 false positive on conditional and other compound expressions

[meziantou]

Fixes #1294

Problem

using System.Globalization;

DateTime? date = null;
_ = "test" + (date.HasValue ? date.Value.ToString(CultureInfo.InvariantCulture) : string.Empty);

This was reported as MA0075 even though the operand is a string.

Cause

Before #1273, MA0075 only inspected concatenation operands that were an implicit conversion to object, and ignored everything else. #1273 changed it to run every operand through CultureSensitiveFormattingContext.GetCultureSensitivity, whose fallback for an unrecognized operation kind was:

// Unknown operation
return CultureSensitivity.CultureSensitive;

IConditionalOperation has no case in that method, so the ternary fell through to "culture-sensitive" regardless of its type. The same false positive applied to coalesce expressions (??), switch expressions and await expressions.

Fix

The fallback now uses the static type of the value, which is what determines the formatting behavior and is already how interpolated string parts are analyzed:

// Unknown operation (conditional expression, coalesce expression, switch expression, await expression, ...).
// The formatting depends on the type of the value, so use it to determine the culture sensitivity.
return GetCultureSensitivity(operation.Type, format: null, instance: operation, options);

"test" + (cond ? s : "") is now clean, while "test" + (cond ? a : b) with double operands is still reported — same for ??, switch and await.

Notes for reviewers

• An operation with a null type (invalid code, method group) now maps to MaybeCultureSensitiveOpaqueRuntimeType instead of CultureSensitive, so it is no longer reported by default. That matches how object-typed values are already treated.
• IInvocationOperation is handled earlier in the method and is unaffected, so MA0011 (UseIFormatProviderAnalyzer) keeps its current behavior for invocations.

Tests

8 tests added to DoNotUseImplicitCultureSensitiveToStringAnalyzerTests (a positive and a negative case for each expression kind, including the exact snippet from the issue).

• Full suite on the default Roslyn version: 3650 passed
• roslyn4.8 (3520), roslyn4.14 (3554), roslyn5.0 (3593), roslyn5.6 (3604): all passed
• dotnet run --project src/DocumentationGenerator: exit 0, no markdown changes