Add general #if conditional compilation block (IfConfig)

[leogdion]

Summary

Currently IfConfig (formerly IfCanImport) only supports #if canImport(Module). It should be expanded into a general-purpose conditional compilation block covering all Swift #if condition types.

Conditions to Support

• .canImport("UIKit") → #if canImport(UIKit) (already works)
• .flag("DEBUG") → #if DEBUG
• .os(.iOS) / .os(.macOS) / .os(.Linux) / .os(.Windows) etc.
• .targetEnvironment(.simulator) / .targetEnvironment(.macCatalyst)
• .swift(>=6.0) — Swift version check
• .compiler(>=6.0) — compiler version check
• .arch(.arm64) / .arch(.x86_64)
• .hasFeature(...) / .hasAttribute(...)
• Boolean combinations: &&, ||, !

Clauses

• #elseif support (additional IfConfigClauseSyntax entries)
• #else support

Related

• #warning("message") and #error("message") could be added as separate CompilerWarning / CompilerError types using PoundDiagnosticDeclSyntax