Do not report MA0184 when the interpolated string is converted to IFormattable

[meziantou]

Problem

MA0184 reported interpolated strings without holes that are converted to System.IFormattable, and its code fix broke the build:

public static System.IFormattable Run() => $"text";
// Fixed code: error CS0029, a string cannot be converted to System.IFormattable
public static System.IFormattable Run() => "text";

Interpolated strings support a conversion to IFormattable (like FormattableString), but a string literal does not. The analyzer only excluded FormattableString.

Changes

• The analyzer now looks at the conversion applied to the interpolated string itself, and does not report when it targets FormattableString or IFormattable, or when the string is used by a custom interpolated string handler.
• This replaces the walk up the parent chain, which looked at every enclosing operation. Only the conversion of the interpolated string itself matters: the conversions of the enclosing operations still apply to the string literal. As a consequence, a string nested in an expression of type FormattableString, such as FormattableString value = Create($"text") where Create takes a string, is now reported, and the fix is valid there.
• docs/Rules/MA0184.md lists IFormattable in the exceptions.

Tests

New tests with an IFormattable target: return expression, local and field assignments, method argument, explicit cast, and conditional branch. New tests checking that the diagnostic and fix still apply for a conversion to object and for the Create($"text") case.

• DoNotUseInterpolatedStringWithoutParametersAnalyzerTests: 20/20 on Roslyn 4.8, 4.14, 5.0, 5.6, and 5.9 (the 5 IFormattable tests failed before the fix)
• Full Roslyn 5.9 test suite: 4387/4387
• dotnet run --project src/DocumentationGenerator: no further changes