Implement #81741 for CoreCLR.

• Add UnsafeAccessorAttribute as a well-known attribute to the runtime [CoreCLR/NativeAOT] UnsafeAccessorAttribute non-generic support #86932
  We will need to extract a named string-value property from the custom attribute - duplicate work item with mono.
• Non-generic scenarios [CoreCLR/NativeAOT] UnsafeAccessorAttribute non-generic support #86932
• Validate generated code can be inlined [CoreCLR/NativeAOT] UnsafeAccessorAttribute non-generic support #86932
• Add runtime testing [CoreCLR/NativeAOT] UnsafeAccessorAttribute non-generic support #86932
• Update Trimmer to respect UnsafeAccessorAttribute
• Validate Diagnostic scenarios

See Mono work item - #86040
See NativeAOT work item - #86438