fix: Fix default value for string-backed enums

[jbelkins]

Description of changes

Fixes code for determining default value for Smithy Enum types (i.e. Swift enum w/ raw value of type String.)

The previous default value code was incorrect (it constructed an enum case name from the raw value when those two values are not necessarily related.) It worked for names that closely match the raw value and do not contain numbers. A pending model change breaks these assumptions, and a compile error results.

Instead, the approach for determining IntEnum raw values is adapted, which is correct - search the Smithy Enum's members to find one whose raw value matches the default value, then use the name of that member to derive the Swift default value.

Scope

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.