[Proposal] C# GUID literal (companion to VB proposal)

[mkane91301]

vblang #27 proposes a syntax for GUID literals and suggests CLR changes to enable GUID constants. This proposal is to match the feature for C# with appropriate syntax.

[dsaf]

We don't even have DateTime literals.

Also RIP dotnet/roslyn#263

[tannergooding]

I would much rather see support for blittable constants first so that literals are actually efficient. Outside of that, calling the constructor that takes a string (or using a code refactoring to convert that to the raw value overload) seems to work well enough.