Provide KeyValuePair.Create static factory method (dotnet/coreclr#5755)

Provide KeyValuePair.Create static factory method (dotnet/coreclr#5755)

Just like Tuple.Create cuts down the noise and annoyance
of having to specify all the Ts when you already have typed
values to provide, a similarly designed KeyValuePair.Create
factory method would make code that creates those pairs
(i.e. public APIs that expose IEnumerable{KeyValuePair}
significantly more concise and readable.

Fixes dotnet/corefxdotnet/coreclr#2127

Commit migrated from https://github.com/dotnet/coreclr/commit/af076482b26baf02f291416e606ad6becbf8d28b