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/check/handle_call_expression.cpp

@@ -48,7 +48,7 @@ auto HandleCallExpr(Context& context, Parse::Node parse_node) -> bool {
     return diagnose_not_callable();
   }
   auto function_decl =
-      context.insts().Get(function_decl_id).TryAs<SemIR::FunctionDeclaration>();
+      context.insts().Get(function_decl_id).TryAs<SemIR::FunctionDecl>();
   if (!function_decl) {
     return diagnose_not_callable();
   }