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

[leogdion]

Shipping the first cut as PoundIf (not IfConfig)

Going to land the first cut under the name PoundIf to match Swift's #-prefixed grammar and to leave the "IfConfig" wording reserved for the SwiftSyntax node type. Public surface exposes a nested PoundIf.Condition enum covering canImport, flag, os, arch, targetEnvironment, swift, compiler, hasFeature, hasAttribute, and boolean combinators (and / or / not), plus string and CodeBlock escape hatches and .elseif / .else modifiers. That matches the condition list in this issue's description.

#if is not a macro expression in SwiftSyntax

Flagging the structural distinction so the eventual API doesn't accidentally try to subsume things it can't represent.

Construct	SwiftSyntax node	Category
#if / #elseif / #else / #endif	IfConfigDeclSyntax (with IfConfigClauseSyntax, poundIfToken, poundElseifToken, poundElseToken, poundEndifToken)	Conditional compilation directive — declaration-level, evaluated at parse/compile time, can wrap arbitrary declarations or statements
#stringify(x), #externalMacro(…), custom freestanding macros, modernized #file / #line / #selector / #keyPath / #colorLiteral / #imageLiteral	MacroExpansionExprSyntax / MacroExpansionDeclSyntax	Macro expansion — expression or declaration, runtime-typed
#available(iOS 17, *), #unavailable(…)	AvailabilityConditionSyntax (inside an if / guard condition)	Availability condition — fragment, not standalone
#sourceLocation(…), #error(…), #warning(…)	PoundSourceLocationSyntax, PoundDiagnosticDeclSyntax (or MacroExpansionDeclSyntax)	Compiler directives

Implications:

• PoundIf covers only the #if family (IfConfigDeclSyntax). It cannot host #stringify or #colorLiteral — those belong in MacroExp (Add MacroExp for freestanding macro expressions #141).
• #available / #unavailable are not standalone #if directives; they're condition elements for If / Guard, so they belong inside the If DSL's condition slot, not inside PoundIf.
• #warning / #error are top-level diagnostic directives — neither PoundIf nor MacroExp. They'd be a separate CompilerWarning / CompilerError DSL (already noted in the "Related" section above).

Cross-ref: #141 tracks MacroExp for MacroExpansionExprSyntax.