Port Encoding.GetEncoding(int) changes from CoreCLR

[justinvp]

Port dotnet/coreclr#6907 from CoreCLR.

Remove EncodingCache and cache UTF32BE in a static field.

[MichalStrehovsky]

We'll only be able to get rid of the TODO once CppCodegen can do interface calls, if my comment from the February pull request is right (I did constrained calls a while ago). We're getting closer, but we're not there yet.

[jkotas]

We'll only be able to get rid of the TODO once CppCodegen

The change is getting rid of the unifier hashtable that contains the constrained calls on this codepath, so it should be fine to remote the TODO on this codepath.

However, it exposed a different problem in CppCodeGen. There is a problem with string literals mangling: __load_string_literal("!"#$%&*;<=>@[]^_{|}");`. The quote inside the string has to be prefixed with``.

[jkotas]

@justinvp Could you please add the ifdef & TODO back? We will look at the CppCodeGen break separately.

[justinvp]

Could you please add the ifdef & TODO back?

@jkotas, done.

[jkotas]

Thanks!

[justinvp]

FYI, I opened #1727 about the string literal issue.