Fix a string interpolation error in CodeGeneration

[ahoppen]

leafProtocolDecl placed two declarations within the same DeclSyntax, which didn’t parse correctly, triggering logStringInterpolationParsingError. Split the function so that one returns the protocol declaration and the other returns the extension.

[ahoppen]

@swift-ci Please test

[Matejkob]

Hmmm, that's interesting. So if we wanna create two functions do we need two different DeclSyntax?

DeclSyntax(
"""
func foo() { }
func bar() { }
"""
)

vs.

DeclSyntax(
"""
func foo() { }
"""
)
DeclSyntax(
"""
func bar() { }
"""
)

?

[ahoppen]

Yes, a DeclSyntax only represents a single declaration. To represent a list of declarations, we should use CodeBlockItemListSyntax or MemberBlockItemListSyntax.