Rename files to use abbreviations (#3389)

Reflects these recent PRs:
* #3375 
* #3382

toolchain/parse/handle_index_expr.cpp

@@ -0,0 +1,26 @@
+// 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
+
+#include "toolchain/lex/token_kind.h"
+#include "toolchain/parse/context.h"
+
+namespace Carbon::Parse {
+
+auto HandleIndexExpr(Context& context) -> void {
+  auto state = context.PopState();
+  state.state = State::IndexExprFinish;
+  context.PushState(state);
+  context.AddNode(NodeKind::IndexExprStart,
+                  context.ConsumeChecked(Lex::TokenKind::OpenSquareBracket),
+                  state.subtree_start, state.has_error);
+  context.PushState(State::Expr);
+}
+
+auto HandleIndexExprFinish(Context& context) -> void {
+  auto state = context.PopState();
+
+  context.ConsumeAndAddCloseSymbol(state.token, state, NodeKind::IndexExpr);
+}
+
+}  // namespace Carbon::Parse