[CMake] Enable library evolution for Darwin overlay.

Fixes rdar://70156840.

Author: varungandhi-apple

cmake/modules/StandaloneOverlay.cmake

@@ -58,9 +58,17 @@ set(SWIFT_NATIVE_CLANG_TOOLS_PATH "${TOOLCHAIN_DIR}/usr/bin" CACHE STRING
 set(SWIFT_NATIVE_SWIFT_TOOLS_PATH "${TOOLCHAIN_DIR}/usr/bin" CACHE STRING
   "Path to Swift tools that are executable on the build machine.")
 
+# NOTE: The initialization in stdlib/CMakeLists.txt will be bypassed if we
+# directly invoke CMake for this directory, so we initialize the variables
+# related to library evolution here as well.
+
+option(SWIFT_STDLIB_STABLE_ABI
+  "Should stdlib be built with stable ABI (library evolution, resilience)."
+  TRUE)
+
 option(SWIFT_ENABLE_MODULE_INTERFACES
   "Generate .swiftinterface files alongside .swiftmodule files."
-  TRUE)
+  "${SWIFT_STDLIB_STABLE_ABI}")
 
 set(SWIFT_STDLIB_BUILD_TYPE "${CMAKE_BUILD_TYPE}" CACHE STRING
   "Build type for the Swift standard library and SDK overlays.")

stdlib/CMakeLists.txt

@@ -42,10 +42,17 @@ endif()
 # User-configurable options for the standard library.
 #
 
+# NOTE: Some of these variables are also initialized in StandaloneOverlay.cmake
+# so that interfaces are emitted when overlays are separately built.
+
 option(SWIFT_STDLIB_STABLE_ABI
   "Should stdlib be built with stable ABI (library evolution, resilience)."
   "${SWIFT_STDLIB_STABLE_ABI_default}")
 
+option(SWIFT_ENABLE_MODULE_INTERFACES
+  "Generate .swiftinterface files alongside .swiftmodule files"
+  "${SWIFT_STDLIB_STABLE_ABI}")
+
 option(SWIFT_ENABLE_COMPATIBILITY_OVERRIDES
   "Support back-deploying compatibility fixes for newer apps running on older runtimes."
   TRUE)
@@ -62,10 +69,6 @@ option(SWIFT_STDLIB_OS_VERSIONING
     "Build stdlib with availability based on OS versions (Darwin only)."
     TRUE)
 
-option(SWIFT_ENABLE_MODULE_INTERFACES
-       "Generate .swiftinterface files alongside .swiftmodule files"
-       "${SWIFT_STDLIB_STABLE_ABI}")
-
 option(SWIFT_COMPILE_DIFFERENTIATION_WITHOUT_TGMATH
        "Build Differentation without tgmath (and dependency on platform runtime libraries)"
        FALSE)