Avoid unnecessary defensive copy for constrained calls on readonly struct receivers

[stevenelliottjr]

Summary

• When calling base class virtual methods (e.g., ToString(), GetHashCode(), Equals()) on readonly struct receivers via constrained. callvirt, the compiler previously used AddressKind.Writeable for the receiver address
• This caused unnecessary defensive copies when the receiver was in a readonly context (e.g., a field accessed through a readonly method's this, or a direct this reference in a readonly struct method)
• Since readonly structs guarantee non-mutation for all their methods, and constrained calls either resolve to a non-mutating method or box the value (which copies it), the original receiver is never mutated
• Changed to use AddressKind.ReadOnly for readonly value type receivers, eliminating the defensive copy

Example (from #76288):

struct X
{
    private Y a;
    public readonly string W() => a.ToString();
}
readonly struct Y { }

Before (21 bytes, 1 local):

ldarg.0
ldfld      "Y X.a"    // load value
stloc.0                // store to temp
ldloca.s   V_0         // address of temp
constrained. "Y"
callvirt   "string object.ToString()"
ret

After (18 bytes, 0 locals):

ldarg.0
ldflda     "Y X.a"    // direct field address
constrained. "Y"
callvirt   "string object.ToString()"
ret

Fixes #76288
Also resolves the optimization noted in #66365

Test plan

• Updated InvokeOnThisBaseMethods test to reflect eliminated defensive copy for ToString() on readonly struct this
• Added new test NoDefensiveCopy_ReadonlyStructField_ConstrainedCall verifying the exact scenario from Unnecessary defensive copy on readonly struct when performing a constrained call #76288
• All 45 readonly struct codegen tests pass
• Full Emit test suite passes (6986 passed, 0 failed)

[AlekseyTs]

// (4,15): warning CS0649: Field 'X.a' is never assigned to, and will always have its default value

Consider suppressing the warning with #pragma

[AlekseyTs]

VerifyDiagnostics

We generally prefer combining VerifyDiagnostics with CompileAndVerify since it collects diagnostics anyway and collects all diagnostics.

[AlekseyTs]

Done with review pass (commit 1)

[AlekseyTs]

@dotnet/roslyn-compiler For a second review

[jjonescz]

Consider marking the issue as closed by the PR (writing literally closes #66365 in the PR's description)