Fix warning for unwrap_or_else in generated builders (#3307)

While attempting to upgrade to Smithy 1.42, I noticed a warning in the
codegen-client integration tests:
```
warning: unnecessary closure used to substitute value for `Option::None`
   --> json_rpc10/rust-client-codegen/src/operation/operation_with_defaults/_operation_with_defaults_output.rs:477:31
    |
477 |             default_int_enum: self.default_int_enum.unwrap_or_else(|| 1),
    |                               ^^^^^^^^^^^^^^^^^^^^^^--------------------
    |                                                     |
    |                                                     help: use `unwrap_or(..)` instead: `unwrap_or(1)`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_lazy_evaluations
```

This change fixes these warnings by switching between unwrap_or_else and
unwrap_or based on the shape.

----

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copy, and redistribute this contribution, under the terms of your
choice._

codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/generators/BuilderGenerator.kt

@@ -407,8 +407,10 @@ class BuilderGenerator(
                         if (default != null) {
                             if (default.isRustDefault) {
                                 rust(".unwrap_or_default()")
+                            } else if (default.complexType) {
+                                rust(".unwrap_or_else(|| #T)", default.expr)
                             } else {
-                                rust(".unwrap_or_else(#T)", default.expr)
+                                rust(".unwrap_or(#T)", default.expr)
                             }
                         } else {
                             withBlock(

codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/generators/DefaultValueGenerator.kt

@@ -6,7 +6,10 @@
 package software.amazon.smithy.rust.codegen.core.smithy.generators
 
 import software.amazon.smithy.model.Model
+import software.amazon.smithy.model.shapes.BooleanShape
+import software.amazon.smithy.model.shapes.EnumShape
 import software.amazon.smithy.model.shapes.MemberShape
+import software.amazon.smithy.model.shapes.NumberShape
 import software.amazon.smithy.model.shapes.SimpleShape
 import software.amazon.smithy.rust.codegen.core.rustlang.Writable
 import software.amazon.smithy.rust.codegen.core.rustlang.rust
@@ -23,17 +26,21 @@ class DefaultValueGenerator(
 ) {
     private val instantiator = PrimitiveInstantiator(runtimeConfig, symbolProvider)
 
-    data class DefaultValue(val isRustDefault: Boolean, val expr: Writable)
+    data class DefaultValue(val isRustDefault: Boolean, val expr: Writable, val complexType: Boolean)
 
     /** Returns the default value as set by the defaultValue trait */
     fun defaultValue(member: MemberShape): DefaultValue? {
         val target = model.expectShape(member.target)
+        val complexType = when (target) {
+            is NumberShape, is EnumShape, is BooleanShape -> false
+            else -> true
+        }
         return when (val default = symbolProvider.toSymbol(member).defaultValue()) {
             is Default.NoDefault -> null
-            is Default.RustDefault -> DefaultValue(isRustDefault = true, writable("Default::default"))
+            is Default.RustDefault -> DefaultValue(isRustDefault = true, writable("Default::default"), complexType)
             is Default.NonZeroDefault -> {
                 val instantiation = instantiator.instantiate(target as SimpleShape, default.value)
-                DefaultValue(isRustDefault = false, writable { rust("||#T", instantiation) })
+                DefaultValue(isRustDefault = false, writable { rust("#T", instantiation) }, complexType)
             }
         }
     }