[6.0] Parse #if canImport as a function call instead of a specialized expression node

[ahoppen]

• Explanation: The parser had special knowledge about how to parse canImport inside #if conditions. This was inconsistent with eg. the parsing of #if swift(<6.0), which gets parsed as a function call. To make it easier to interpret the version passed to canImport(MyModule, _version: <#version#>), add ExprSyntax.interpretedAsVersionTuple that takes eg. 1.2.3, which got parsed as a member access 3 to the 1.2 float literal and re-parses it as a VersionTupleSyntax. This relaxation also allows string literals as version parameters to #if canImport.
• Scope: Parsing of canImport directives in #if conditions.
• Risk: Low, we’ve to quite a few tests for this, function calls are more permissive than CanImportExprSyntax and it is unlikely that anyone is relying on the interpretation of the version number
• Testing: Updated the existing test cases
• Issue: rdar://121070235, Remove CanImportExprSyntax and provide API to interpret an ExprSyntax as a VersionTupleSyntax #1972
• Reviewer: @bnbarham and @rintaro on Remove CanImportExprSyntax and provide API to interpret an ExprSyntax as a VersionTupleSyntax #2025

[ahoppen]

@swift-ci Please test

[ahoppen]

@swift-ci Please test Windows