Consolidates async calling convention detection logic into a single MethodDesc extension method, removing the need for MethodSignatureFlags.AsyncCall flag.

Changes:

• Added IsAsyncCall extension method on MethodDesc that unifies detection logic: returns true for async variants and async intrinsics that don't return Task/ValueTask
• Removed MethodSignatureFlags.AsyncCall enum value and MethodSignature.IsAsyncCall property
• Updated call sites in CorInfoImpl.cs to use the new extension method for CORINFO_CALLCONV_ASYNCCALL and CORJIT_FLAG_ASYNC
• Simplified AsyncMethodVariant to no longer set the AsyncCall flag in signature

Rationale:

Async calling convention is now determined at the MethodDesc level (similar to hidden parameters in generic calling conventions) rather than being stored in signature flags. This properly handles both async variants (which have the flag set by signature transformation) and async intrinsics (which have IsAsync metadata but don't return Task/ValueTask).

// Before: Logic scattered across multiple locations
if (method.IsAsync && !method.Signature.ReturnsTaskOrValueTask())
    sig->callConv |= CorInfoCallConv.CORINFO_CALLCONV_ASYNCCALL;

// After: Unified in extension method
if (method.IsAsyncCall())
    sig->callConv |= CorInfoCallConv.CORINFO_CALLCONV_ASYNCCALL;

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