Reserve operator method names

[RexJaeschke]

While the mapping of C# operators to compiler-generated method names is not stated in the C# spec, it is in the CLI spec. (Partition I, I.4 and I.5.) Unfortunately, this is insufficient, for the following reasons:

1. The C# spec normatively references Partition IV only: ISO/IEC 23271:2012, Common Language Infrastructure (CLI), Partition IV: Base Class Library (BCL), Extended Numerics Library, and Extended Array Library. In the C# spec Introduction we say, “Although Microsoft’s implementation of C# relies on CLI for library and run-time support, other implementations of C# need not, provided they support an alternate way of getting at the minimum CLI features required by this C# standard (see Annex C).” And Annex C is all about the library in Partition IV. So, we can’t simply defer to Partition I without making it a Normative Reference (which I’m sure we don’t want to do).
2. In any event, those tables do not actually show the C# mapping! Instead, their right-most columns imply a mapping to C++ and are marked as informative.
3. Even if we could defer to the CLI for this information, we’d still have to augment it for the new operator function names resulting from V11’s “Checked user-defined operators” as well as >>>.

This PR replicates the tables from the CLI spec, replacing C++ with C# in the (now normative) right column, and puts them in a new section, 15.3.10.6, “Member names reserved for operators.” And as we already know the extra names that V11 uses, I’ve put those in the current spec, but tagged with “reserved for future use” to warn people to stay away from defining methods with those names.

[BillWagner]

This LGTM @RexJaeschke

I'd like Jon or Nigel to weigh in, and then let's ship it.

[jskeet]

Approved with the change to "Method names"