[SwiftCompiler/Regex] Use bridged DiagnosticEngine for error reporting

This fixes:
 * An issue where the diagnostic messages were leaked
 * Diagnose at correct position inside the regex literal

To do this:
 * Introduce 'Parse' SwiftCompiler module that is a bridging layer
   between '_CompilerRegexParser' and C++ libParse
 * Move libswiftParseRegexLiteral and libswiftLexRegexLiteral to 'Parse'

Also this change makes 'SwiftCompilerSources/Package.swift' be configured
by CMake so it can actually be built with 'swift-build'.

rdar://92187284

SwiftCompilerSources/Package.swift

@@ -1,51 +0,0 @@
-// swift-tools-version:5.3
-
-import PackageDescription
-
-let package = Package(
-  name: "SwiftCompilerSources",
-  platforms: [
-    .macOS("10.9"),
-  ],
-  products: [
-    .library(
-      name: "Swift",
-      type: .static,
-      targets: ["SIL", "Optimizer", "_CompilerRegexParser"]),
-  ],
-  dependencies: [
-  ],
-  // Note that all modules must be added to LIBSWIFT_MODULES in the top-level
-  // CMakeLists.txt file to get debugging working.
-  targets: [
-    .target(
-      name: "SIL",
-      dependencies: [],
-      swiftSettings: [SwiftSetting.unsafeFlags([
-          "-I", "../include/swift",
-          "-cross-module-optimization"
-        ])]),
-    .target(
-      name: "_CompilerRegexParser",
-      dependencies: [],
-      path: "_RegexParser",
-      swiftSettings: [
-        .unsafeFlags([
-          "-I", "../include/swift",
-          "-cross-module-optimization",
-        ]),
-        // Workaround until `_RegexParser` is imported as implementation-only
-        // by `_StringProcessing`.
-        .unsafeFlags([
-          "-Xfrontend",
-          "-disable-implicit-string-processing-module-import"
-        ])]),
-    .target(
-      name: "Optimizer",
-      dependencies: ["SIL", "_CompilerRegexParser"],
-      swiftSettings: [SwiftSetting.unsafeFlags([
-          "-I", "../include/swift",
-          "-cross-module-optimization"
-        ])]),
-  ]
-)