[cmake] Add private build extension points

Adding vendor-specific extension points to the build system, giving
vendors a place to put their internal settings and defaults, without
having to modify the build directly. This helps reduce the chances of
merge conflicts and public changes breaking internal settings.

The location of the macros are set with `SwiftCore_PRIVATE_MODULE_DIR`,
so vendors can keep their private extensions in a separate location if
desired. Otherwise, it defaults to `cmake/modules/private`, which
intentionally does not exist at this time.

Currently, we have extensions for the default settings and global
settings.

Author: etcwilde

Runtimes/Core/CMakeLists.txt

@@ -31,7 +31,8 @@
 
 cmake_minimum_required(VERSION 3.26...3.29)
 
-list(APPEND CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake/modules")
+set(SwiftCore_CMAKE_MODULES_DIR "${CMAKE_SOURCE_DIR}/cmake/modules")
+list(APPEND CMAKE_MODULE_PATH ${SwiftCore_CMAKE_MODULES_DIR})
 include(CMakeWorkarounds)
 project(SwiftCore LANGUAGES C CXX Swift VERSION 6.1)
 
@@ -47,6 +48,13 @@ set(SwiftCore_SWIFTC_SOURCE_DIR
   "${PROJECT_SOURCE_DIR}/../../"
   CACHE FILEPATH "Path to the root source directory of the Swift compiler")
 
+# Hook point for vendor-specific extensions to the build system
+# Allowed extension points:
+#   - DefaultSettings.cmake
+#   - Settings.cmake
+set(SwiftCore_VENDOR_MODULE_DIR "${SwiftCore_CMAKE_MODULES_DIR}/vendor"
+  CACHE FILEPATH "Location for private build system extension")
+
 include(GNUInstallDirs)
 include(AvailabilityMacros)
 include(CompilerSettings)
@@ -55,6 +63,8 @@ include(EmitSwiftInterface)
 include(PlatformInfo)
 include(gyb)
 
+include("${SwiftCore_VENDOR_MODULE_DIR}/Settings.cmake" OPTIONAL)
+
 defaulted_option(SwiftCore_ENABLE_LIBRARY_EVOLUTION "Generate ABI resilient runtime libraries")
 
 defaulted_option(SwiftCore_ENABLE_CRASH_REPORTER_CLIENT "Enable Apple CrashReporter integration")

Runtimes/Core/cmake/modules/DefaultSettings.cmake

@@ -43,3 +43,5 @@ elseif(LINUX OR ANDROID OR BSD)
 elseif(WIN32)
   set(SwiftCore_OBJECT_FORMAT_default "coff")
 endif()
+
+include("${SwiftCore_VENDOR_MODULE_DIR}/DefaultSettings.cmake" OPTIONAL)