Build _RegexParser without implicit import of _StringProcessing

Explicitly ask the compiler not to implicitly import _StringProessing. This is to avoid a circular dependency when `-enable-experimental-string-processing` is enabled by default.

Author: rxwei

Package.swift

@@ -38,6 +38,7 @@ let package = Package(
             dependencies: [],
             swiftSettings: [
                 .unsafeFlags(["-enable-library-evolution"]),
+                .unsafeFlags(["-Xfrontend", "-disable-implicit-string-processing-module-import"]),
                 availabilityDefinition
             ]),
         .testTarget(