Prepare to enable string processing by default.

This is some groundwork to make it possible to flip the `-enable-experimental-string-processing` flag on by default if and when it passes Swift Evolution. This PR itself do **not** change the defaults.

- Do not implicitly import _StringProcessing when building a module interface.
- Do not implicitly import _StringProcessing when core libraries, same as _Concurrency.

Author: rxwei

lib/Frontend/Frontend.cpp

@@ -794,7 +794,9 @@ bool CompilerInvocation::shouldImportSwiftConcurrency() const {
 
 bool CompilerInvocation::shouldImportSwiftStringProcessing() const {
   return getLangOptions().EnableExperimentalStringProcessing &&
-      !getLangOptions().DisableImplicitStringProcessingModuleImport;
+      !getLangOptions().DisableImplicitStringProcessingModuleImport &&
+      getFrontendOptions().InputMode !=
+        FrontendOptions::ParseInputMode::SwiftModuleInterface;
 }
 
 /// Implicitly import the SwiftOnoneSupport module in non-optimized

stdlib/cmake/modules/AddSwiftStdlib.cmake

@@ -1766,6 +1766,12 @@ function(add_swift_target_library name)
                       "-Xfrontend;-disable-implicit-distributed-module-import")
   endif()
 
+  # Turn off implicit import of _StringProcessing when building libraries
+  if(SWIFT_ENABLE_EXPERIMENTAL_STRING_PROCESSING)
+    list(APPEND SWIFTLIB_SWIFT_COMPILE_FLAGS
+                      "-Xfrontend;-disable-implicit-string-processing-module-import")
+  endif()
+
   if(SWIFTLIB_IS_STDLIB AND SWIFT_STDLIB_ENABLE_PRESPECIALIZATION)
     list(APPEND SWIFTLIB_SWIFT_COMPILE_FLAGS "-Xfrontend;-prespecialize-generic-metadata")
   endif()