New name for "constant" value phase

[josh11b]

(separate from the question of whether the value phases are actually just value categories)

The thing we want to name is "a value that is known at type-checking time" in contrast to "symbolic value", which is a value that will be known at code generation / monomorphization time but type checking is done without knowing its value, or "runtime value" which is not known until runtime.

Original suggestion for this name in #typesystem on Discord. Suggestion to find a new name in PR #1378.

[mconst]

I've been warming up to the idea of referring to all :! values as "constants", and the ones with template :! as "template constants". Some things I like about this nomenclature:

• it exactly matches our syntax
• it matches people's intuition that a constant is something known at compile time
• it correctly implies that template constants are more specialized, and non-template constants are the normal thing you should use in ordinary code
• these names are natural enough that I think users will try to use them, whether they're our official terms or not! As @chandlerc pointed out in https://github.com/carbon-language/carbon-lang/pull/1378/files#r921592116, users will want to refer to all :! values as constants, so it's great if we can make that the actual name.

[chandlerc]

I've been warming up to the idea of referring to all :! values as "constants", and the ones with template :! as "template constants".

Slight tweak that preserves many of the benefits that I want to mention:

:! -> "symbolic constants"

template _:! -> "template constants"

The idea being that they are both constants, symbolic ones unavailable at type-checking, template available.

I'm not advocating one way or the other, just floating it so we can think about it. I'm still pondering a bit.

[chandlerc]

Just confirming that the leads are happy with the tweaked names here:

#1391 (comment)

Marking this as decided!

[chandlerc]

Just a quick reminder to not use specific issues to post more general feedback -- that should happen on its own discussion thread. Also, please keep feedback constructive.