Fix MA0065/MA0066 never checking for a GetHashCode override

[meziantou]

What

DoNotUseDefaultEqualsOnValueTypeAnalyzer resolved ValueTypeGetHashCodeSymbol with nameof(ValueType.Equals) instead of nameof(ValueType.GetHashCode):

ValueTypeEqualsSymbol      = ValueTypeSymbol.GetMembers(nameof(ValueType.Equals))...;
ValueTypeGetHashCodeSymbol = ValueTypeSymbol.GetMembers(nameof(ValueType.Equals))...;  // bug

Both fields held the same symbol, so the two checks in HasDefaultEqualsOrHashCodeImplementations both tested for an Equals override and GetHashCode was never looked at.

Why it matters

A struct that overrides Equals but not GetHashCode still uses the reflection-based ValueType.GetHashCode — precisely the performance problem MA0065 and MA0066 exist to catch — yet was reported by neither rule:

#pragma warning disable CS0659
struct Test { public override bool Equals(object o) => throw null; }

_ = new Test().GetHashCode();                              // was missing MA0065
_ = new System.Collections.Generic.Dictionary<Test, int>(); // was missing MA0066

Every such struct escaped both rules. The Equals half of the check was never broken, so this is purely a set of missed diagnostics — no false positives were being produced.

Tests

Added to DoNotUseDefaultEqualsOnValueTypeAnalyzerTests:

• GetHashCode_OnlyEqualsOverriden — MA0065 on a GetHashCode() call
• Constructor_OnlyEqualsOverriden — MA0066 on HashSet<T> / Dictionary<TKey, TValue>
• Equals_OnlyGetHashCodeOverriden — guards the symmetric case (passed before the fix too)

Verification

• Reverting the fix makes 3 of the 4 new tests fail, confirming they pin the bug.
• With the fix, 44/44 tests in the class pass on all five Roslyn versions (4.8, 4.14, 5.0, 5.6, 5.9).
• Full suite on the default version (roslyn5.9): 3913/3913 pass.
• dotnet run --project src/DocumentationGenerator exits 0 with no markdown changes. The MA0065/MA0066 docs already describe checking both Equals and GetHashCode, so the fix brings the implementation in line with the documented behavior and no doc update was needed.

Reviewer note

This makes the rules report in cases they previously stayed silent on, so consumers with structs that override only Equals may see new MA0065/MA0066 warnings after upgrading. That is the intended behavior of both rules.