Use abbreviation "decl" instead of "declaration" (#3382)

Part of switching to the [abbreviations we've decided to
use](https://docs.google.com/document/d/1RRYMm42osyqhI2LyjrjockYCutQ5dOf8Abu50kTrkX0/edit?resourcekey=0-kHyqOESbOHmzZphUbtLrTw#heading=h.pph7i5m5un7q).

I will rename files in a follow-up PR.

toolchain/parse/handle_statement.cpp

@@ -98,9 +98,9 @@ auto HandleStatementForHeader(Context& context) -> void {
     context.PushState(state);
     context.PushState(State::VarAsFor);
   } else {
-    CARBON_DIAGNOSTIC(ExpectedVariableDeclaration, Error,
+    CARBON_DIAGNOSTIC(ExpectedVariableDecl, Error,
                       "Expected `var` declaration.");
-    context.emitter().Emit(*context.position(), ExpectedVariableDeclaration);
+    context.emitter().Emit(*context.position(), ExpectedVariableDecl);
 
     if (auto next_in = context.FindNextOf({Lex::TokenKind::In})) {
       context.SkipTo(*next_in);