[SwiftIfConfig] Add StaticBuildConfiguration

Introduce StaticBuildConfiguration, a data structure that describes all
of the aspects of a build configuration that are needed for `#if`
evaluation. It is Codable so that it can be stored and replayed later.

Author: DougGregor

Sources/SwiftIfConfig/BuildConfiguration.swift

@@ -14,7 +14,7 @@ import SwiftSyntax
 
 /// Describes the ordering of a sequence of bytes that make up a word of
 /// storage for a particular architecture.
-public enum Endianness: String {
+public enum Endianness: String, Codable {
   /// Little endian, meaning that the least significant byte of a word is
   /// stored at the lowest address.
   case little

Sources/SwiftIfConfig/CMakeLists.txt

@@ -17,6 +17,7 @@ add_swift_syntax_library(SwiftIfConfig
   IfConfigDiagnostic.swift
   IfConfigEvaluation.swift
   IfConfigFunctions.swift
+  StaticBuildConfiguration.swift
   SyntaxLiteralUtils.swift
   SyntaxProtocol+IfConfig.swift
   VersionTuple+Parsing.swift