Clean up some lambda conversion operator code, NFC

We don't need special handling in CodeGenFunction::GenerateCode for
lambda block pointer conversion operators anymore. The conversion
operator emission code immediately calls back to the generic
EmitFunctionBody.

Rename EmitLambdaStaticInvokeFunction to EmitLambdaStaticInvokeBody for
better consistency with the other Emit*Body methods.

I'm preparing to do something about PR28299, which touches this code.

llvm-svn: 310145