Make Utf16CharMarshaller pin ref parameters

[elinor-fung]

Make our char marshaller pin ref/out instead of copy just the char to a local.

ref char parameters are being used as string buffers in parts of the libraries.

Before:

internalstaticunsafepartialcharMethod(charp,incharpIn,refcharcRef,outcharpOut){
unsafe
{ushort__p_gen_native=default;ushort__pIn_gen_native=default;ushort__cRef_gen_native=default;pOut=default;ushort__pOut_gen_native=default;char__retVal=default;ushort__retVal_gen_native=default;//// Marshal//__p_gen_native=p;__pIn_gen_native=pIn;__cRef_gen_native=cRef;__retVal_gen_native=__PInvoke__(__p_gen_native,&__pIn_gen_native,&__cRef_gen_native,&__pOut_gen_native);//// Unmarshal//__retVal=(char)__retVal_gen_native;pOut=(char)__pOut_gen_native;cRef=(char)__cRef_gen_native;return__retVal;}[System.Runtime.InteropServices.DllImportAttribute("DNE",EntryPoint="Method",CharSet=System.Runtime.InteropServices.CharSet.Unicode)]externstaticunsafeushort__PInvoke__(ushortp,ushort*pIn,ushort*cRef,ushort*pOut);}

After:

internalstaticunsafepartialcharMethod(charp,incharpIn,refcharcRef,outcharpOut){
unsafe
{ushort__pIn_gen_native=default;pOut=default;char__retVal=default;ushort__retVal_gen_native=default;//// Marshal//__pIn_gen_native=pIn;
fixed (char*cRef__pinned=&cRef){
fixed (char*pOut__pinned=&pOut)__retVal_gen_native=__PInvoke__((ushort)p,&__pIn_gen_native,(ushort*)cRef__pinned,(ushort*)pOut__pinned);}//// Unmarshal//__retVal=(char)__retVal_gen_native;return__retVal;}[System.Runtime.InteropServices.DllImportAttribute("DNE",EntryPoint="Method",CharSet=System.Runtime.InteropServices.CharSet.Unicode)]externstaticunsafeushort__PInvoke__(ushortp,ushort*pIn,ushort*cRef,ushort*pOut);}

cc @AaronRobinsonMSFT@jkoritzinsky

[AaronRobinsonMSFT]

@elinor-fung The in char pIn is still a by-ref so it should also get the pinned treatment I believe.

[elinor-fung]

I didn't do it for in because one of the tests checks that the value doesn't get updated, but it looks like the test was doing the wrong thing (double-checked built-in). Will update.

[elinor-fung]

Updated. Also added something to the compat doc about in for blittable types and Unicode char.