Preserve defaultValue literals

Fixes graphql#3051

This change solves the problem of default values defined via SDL not always resolving correctly through introspection by preserving the original GraphQL literal in the schema definition. This changes argument and input field definitions `defaultValue` field from just the "value" to a new `GraphQLDefaultValueUsage` type which contains either or both "value" and "literal" fields.

Since either of these fields may be set, new functions for resolving a value or literal from either have been added - `getLiteralDefaultValue` and `getCoercedDefaultValue` - these replace uses that either assumed a set value or were manually converting a value back to a literal.

Here is the flow for how a default value defined in an SDL would be converted into a functional schema and back to an SDL:

**Before this change:**

```
(SDL) --parse-> (AST) --coerceInputLiteral--> (defaultValue config) --valueToAST--> (AST) --print --> (SDL)
```

`coerceInputLiteral` performs coercion which is a one-way function, and `valueToAST` is unsafe and set to be deprecated in graphql#3049.

**After this change:**

```
(SDL) --parse-> (defaultValue literal config) --print --> (SDL)
```

src/utilities/__tests__/coerceInputValue-test.ts

@@ -892,3 +892,26 @@ describe('coerceDefaultValue', () => {
     expect(parseValueCalls).to.deep.equal(['hello']);
   });
 });
+
+describe('coerceDefaultValue', () => {
+  it('memoizes coercion', () => {
+    const parseValueCalls: any = [];
+
+    const spyScalar = new GraphQLScalarType({
+      name: 'SpyScalar',
+      parseValue(value) {
+        parseValueCalls.push(value);
+        return value;
+      },
+    });
+
+    const defaultValueUsage = {
+      literal: { kind: Kind.STRING, value: 'hello' },
+    } as const;
+    expect(coerceDefaultValue(defaultValueUsage, spyScalar)).to.equal('hello');
+
+    // Call a second time
+    expect(coerceDefaultValue(defaultValueUsage, spyScalar)).to.equal('hello');
+    expect(parseValueCalls).to.deep.equal(['hello']);
+  });
+});

src/utilities/coerceInputValue.ts

@@ -408,3 +408,21 @@ export function coerceDefaultValue(
   }
   return coercedValue;
 }
+
+/**
+ * @internal
+ */
+export function coerceDefaultValue(
+  defaultValue: GraphQLDefaultValueUsage,
+  type: GraphQLInputType,
+): unknown {
+  // Memoize the result of coercing the default value in a hidden field.
+  let coercedValue = (defaultValue as any)._memoizedCoercedValue;
+  if (coercedValue === undefined) {
+    coercedValue = defaultValue.literal
+      ? coerceInputLiteral(defaultValue.literal, type)
+      : defaultValue.value;
+    (defaultValue as any)._memoizedCoercedValue = coercedValue;
+  }
+  return coercedValue;
+}