[Staging] Force stdlib into “Swift 6 mode” for #file

We ultimately want to explicitly change standard library uses of #file to #fileID, but once we do, previous compilers won’t be able to build the standard library. So instead, we will temporarily build the standard library with -enable-experimental-concise-pound-file, which should have the same effect, but will back-deploy to compilers going back several months.

Author: beccadax

stdlib/public/core/CMakeLists.txt

@@ -293,6 +293,9 @@ if(SWIFT_STDLIB_ENABLE_STDLIBCORE_EXCLUSIVITY_CHECKING)
   list(APPEND swift_stdlib_compile_flags "-enforce-exclusivity=checked")
 endif()
 
+# STAGING: Temporarily avoids having to write #fileID in Swift.swiftinterface.
+list(APPEND swift_stdlib_compile_flags "-Xfrontend" "-enable-experimental-concise-pound-file")
+
 if(SWIFT_CHECK_ESSENTIAL_STDLIB)
   add_swift_target_library(swift_stdlib_essential ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_STDLIB IS_STDLIB_CORE
       INSTALL_IN_COMPONENT never_install

test/SILGen/magic_identifier_file.swift

@@ -53,7 +53,7 @@ func swift6LibraryUse() {
 func stdlibUse() {
 // BOTH-LABEL: sil {{.*}} @$s3Foo9stdlibUseyyF
   fatalError()
-// BOTH: string_literal utf8 "SOURCE_DIR/test/SILGen/magic_identifier_file.swift"
+// BOTH: string_literal utf8 "Foo/magic_identifier_file.swift"
 }
 
 func forceUnwrap(_ x: ()?) {