Fix convertExamplesInObjectSpec to avoid in-place modification (#3108)

Due to an unfortunate interaction, pulumi/pulumi-aws on commit
7c14e422b23f9fbb34e37e381adb9d9ee4da49b4 started generating
`{convertExamples:123}` markers into the schema. These markers are an
artifact of convert_cli.go implementation and should not leak.

The root cause is subtle, with type reuse introduced in pulumi-aws,
convertExamplesInObjectSpec gets to repeatedly traverse the same
in-memory location, which causes cliConverter.StartConvertingExamples()
to call itself:

    docs = StartConvertingExamples(StartConvertingExamples(docs, p), p)

Which breaks the assumptions, as it now assumes `{convertExamples:%d}`
is an HCL snippet.

The simple fix here is to avoid self-modifying the schema during
convertExamplesInObjectSpec. This is the expected behavior with the
signature:

```
    func (g *Generator) convertExamplesInObjectSpec(path examplePath, spec pschema.ObjectTypeSpec) pschema.ObjectTypeSpec {
```

If later this changes to in-place traversal for efficiency, the
signature should also change to not return ObjectTypeSpec.

Author: t0yv0

pkg/tfgen/generate_schema.go

@@ -1114,9 +1114,12 @@ func (g *Generator) convertExamplesInObjectSpec(
 	path examplePath, spec pschema.ObjectTypeSpec,
 ) pschema.ObjectTypeSpec {
 	spec.Description = g.convertExamples(spec.Description, path)
+
+	newProperties := map[string]pschema.PropertySpec{}
 	for name, prop := range spec.Properties {
-		spec.Properties[name] = g.convertExamplesInPropertySpec(path.Property(name), prop)
+		newProperties[name] = g.convertExamplesInPropertySpec(path.Property(name), prop)
 	}
+	spec.Properties = newProperties
 	return spec
 }