Cherry-pick of #80547 for the 6.2 release branch.
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Explanation: This cherry picks the implementation of SE-0477 to add a
string interpolation method with a `default:` parameter for optional
interpolation values.
Main Branch PR: https://github.com/swiftlang/swift/pull/80547
Risk: Low.
Reviewed By: @stephentyrone
Resolves: rdar://150865613
Testing: New tests for the string interpolations and fix-its.
In 'protocol', 'Self' is implicitly declared as a generic parameter.
In 'struct' and 'enum', 'Self' is just an alias for the nominal type.
In 'class', 'Self' is a dynamic type and usable only in result types and
inside function bodies.
rdar://problem/61307680
