Expression phase terminology (#2964)

Update terminology around expression properties:

-   "value phase" -> "expression phase"
-   "symbolic value" -> "symbolic constant"
-   "constant" -> "template constant"
-   "constant or symbolic value" -> "constant"

Implements the decision in #1391 .

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Co-authored-by: jonmeow <jperkins@google.com>

proposals/p2964.md

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+# Expression phase terminology
+
+<!--
+Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+Exceptions. See /LICENSE for license information.
+SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+-->
+
+[Pull request](https://github.com/carbon-language/carbon-lang/pull/2964)
+
+<!-- toc -->
+
+## Table of contents
+
+-   [Abstract](#abstract)
+-   [Problem](#problem)
+-   [Background](#background)
+-   [Proposal](#proposal)
+-   [Rationale](#rationale)
+-   [Alternatives considered](#alternatives-considered)
+
+<!-- tocstop -->
+
+## Abstract
+
+Update terminology around expression properties:
+
+-   "value phase" -> "expression phase"
+-   "symbolic value" -> "symbolic constant"
+-   "constant" -> "template constant"
+-   "constant or symbolic value" -> "constant"
+
+Implements the decision in
+[#1391](https://github.com/carbon-language/carbon-lang/issues/1391).
+
+## Problem
+
+There are a few concerns with the current terminology:
+
+-   We were not happy with it at the time it was introduced in
+    [proposal #1378](https://github.com/carbon-language/carbon-lang/pull/1378/files/198c7bd152bc4e223f1fc2484455e17ba31f19f9#r921592116),
+    but didn't want to block that proposal on finding better names.
+-   It is a property of expressions, not values. We are switching from "value
+    categories" to "expression categories" for the same reason. See
+    [proposal #2006](https://github.com/carbon-language/carbon-lang/pull/2006)
+    and [PR #2744](https://github.com/carbon-language/carbon-lang/pull/2744).
+-   No good term for the bindings that use `:!`, which share a number of
+    properties, and often we want to refer to those together.
+
+The concerns about the names of the individual phases led to
+[questions-for-leads issue #1391: New name for "constant" value phase](https://github.com/carbon-language/carbon-lang/issues/1391).
+This proposal implements the resolution from that issue.
+
+## Background
+
+The current names were introduced in
+[proposal #1378: Design overview update part 7: values](https://github.com/carbon-language/carbon-lang/pull/1378)
+based on a discussion in
+[#typesystem on Discord](https://discord.com/channels/655572317891461132/708431657849585705/996547601451204630).
+I am not aware of corresponding terminology in other programming languages.
+
+## Proposal
+
+We update terminology around expression properties, specifically what we
+previously referred to as "value phase" and values it can have, as follows:
+
+-   "value phase" -> "expression phase": since this is a property of
+    expressions, not values.
+-   "symbolic value" -> "symbolic constant": for symbolic compile-time values
+    like checked-generic parameters.
+-   "constant" -> "template constant": for compile-time values where the value
+    is available during type checking, like literals and `template` parameters.
+
+By making these last two terms both end in "constant," we allow their
+combination (including all compile-time values) to be collectively referred to
+using the term "constant." For example, either kind of constant may be passed to
+a function with either kind of constant binding.
+
+## Rationale
+
+The goals of this proposal are:
+
+-   clearer and more concise communication, and
+-   making it easier for people to learn Carbon by using more consistent
+    terminology.
+
+This supports Carbon's goal of having
+[Code that is easy to read, understand, and write](/docs/project/goals.md#code-that-is-easy-to-read-understand-and-write).
+
+## Alternatives considered
+
+We were happy with this option and did not spend time coming up with more
+alternatives.