Fix MA0075 false positive on FormattableString.Invariant

[meziantou]

Fixes #1321

Problem

Since 3.0.150 (#1273), MA0075 reports FormattableString.Invariant($"...") when the result is used directly as an operand of a string concatenation:

_ = System.FormattableString.Invariant($"abc{1:N0}") + b; // MA0075

Storing the result in a local first was already accepted, so the inline form should be too.

Cause

CultureSensitiveFormattingContext.GetCultureSensitivity calls GetInterpolatedStringArgumentCultureSensitivity for invocations, which treats any invocation with a FormattableString (or interpolated string handler) argument as producing the culture sensitivity of that argument, and returns before anything else is considered. For Invariant($"abc{1:N0}") the argument is culture sensitive, so the whole invocation was classified as culture sensitive — even though Invariant exists precisely to format that argument with CultureInfo.InvariantCulture.

Fix

Add System.FormattableString.Invariant(System.FormattableString) to CreateExcludedMethods, which is consulted before the interpolated string argument path.

I used the excluded-methods list rather than skipping every method whose containing type is System.FormattableString, because FormattableString.CurrentCulture lives on the same type and is genuinely culture sensitive — it must keep being reported. FormattableString.ToString(IFormatProvider) and string.Create(IFormatProvider, ...), also mentioned in the issue, were already covered by the existing HasArgumentOfType(FormatProviderSymbol) check, so they needed no change.

Tests

• FormattableString_Invariant_StringConcat covers the four concatenation forms from the issue (left operand, right operand, both operands, parenthesized).
• FormattableString_CurrentCulture_StringConcat locks in that FormattableString.CurrentCulture is still reported.

Verification

• Confirmed the new test fails on the unpatched analyzer and passes with the fix.
• Full test suite passes on roslyn5.9 (3774 tests) and roslyn4.8 (3643 tests).
• dotnet run --project src/DocumentationGenerator exits 0 with no markdown changes.