[GR-50773] Improve typing of GraphProvider and signatures.

[graalvmbot]

Now that ParseOnce is the only execution mode, and ResolvedSignature can provide better typing for signatures, this PR improves typing and removes explicit type checks.

• In GraphProvider.buildGraph, pass in AnalysisMethod to remove later explicit casts.
• Remove SubstrateGraphKit.loadArguments. It is just silly to load arguments based on the method signature (which could be the wrong signature), we can just collect the arguments when the FrameStateBuilder creates them.
• Return a typed signature from AnalysisMethod.getSignature and HostedMethod.getSignature and remove later explicit casts.
• Use the getParameterType and getReturnType methods of ResolvedSignature that do not need an accessingClass parameter.
• Use ResolvedSignature.toParameterList where appropriate and remove later explicit casts.
• Replace SimpleSignature with ResolvedSignature
• Do not use ResolvedJavaMethod.toParameterTypes because it is badly defined for constructors. Overrides in AnalysisMethod and HostedMethod prevent future usage in our universe.