Add nullable type reflection cache

[israellot]

Deserializing a class like the one bellow has a performance hit presumably due to calls to GetGenericTypeDefinition and Nullable.GetUnderlyingType.
This is aggravated when using converters like StringEnumConverter which add a nullable type check at CanConvert method

public class NullablePropertiesTestClass
{
    public int? a { get; set; }            
    public TestEnum? b { get; set; }
    public bool? c { get; set; }

    public enum TestEnum
    {
        a,
        b,
        c
    }
}

This PR opens up the discussion of adding a simple cache for these expensive reflection calls.
On this particular example, around 30% gains at the expense of the memory for holding the cache.
Benchmark code is included.

[sungam3r]

Suggested change

	lock(NullableInfoCache){
	lock(NullableInfoCache) {

[sungam3r]

Suggested change

	var isNullable = (genericTypeDefinition == typeof(Nullable<>));
	var isNullable = genericTypeDefinition == typeof(Nullable<>);

[israellot]

dotnet/runtime#73860
Related discussion

[MazeMonster]

License