CMake: Set language version across supplemental libs

The Swift language version appears in the Swift interface and impacts
how the interface is handled by the compiler when importing the module.
The version only appears if it is set, which means that downstream
projects setting the version explicitly may interpret the swift
interface file differently.

This also factors the setting out into a separate file, which is used to
set the C++ standard and Swift language version since it is the same
across these libraries at this time. If this assumption changes, we can
refactor as necessary and this structure shouldn't be seen as
enforcement.

Finally, this patch moves setting the language version and standard
variables before the languages are enabled. This ensures that the
try-compile applies the appropriate flags when verifying that the
compiler works.

Author: etcwilde

Runtimes/Supplemental/Distributed/CMakeLists.txt

@@ -13,6 +13,7 @@ list(APPEND CMAKE_MODULE_PATH
   "${CMAKE_SOURCE_DIR}/../cmake/modules"
   "${CMAKE_SOURCE_DIR}/../../cmake/modules")
 
+include(LanguageVersions)
 include(SwiftProjectVersion)
 project(SwiftDistributed
   LANGUAGES C CXX Swift
@@ -22,17 +23,10 @@ if(NOT PROJECT_IS_TOP_LEVEL)
   message(SEND_ERROR "Swift Distributed must build as a standalone project")
 endif()
 
-set(CMAKE_Swift_LANGUAGE_VERSION 5)
-
 set(CMAKE_POSITION_INDEPENDENT_CODE YES)
 
 set(CMAKE_C_VISIBILITY_PRESET "hidden")
-
-set(CMAKE_CXX_EXTENSIONS NO)
-set(CMAKE_CXX_STANDARD 17 CACHE STRING "C++ standard to conform to")
-set(CMAKE_CXX_STANDARD_REQUIRED YES)
 set(CMAKE_CXX_VISIBILITY_PRESET "hidden")
-
 set(CMAKE_VISIBILITY_INLINES_HIDDEN YES)
 
 set(${PROJECT_NAME}_SWIFTC_SOURCE_DIR

Runtimes/Supplemental/Observation/CMakeLists.txt

@@ -13,6 +13,7 @@ list(APPEND CMAKE_MODULE_PATH
   "${CMAKE_SOURCE_DIR}/../cmake/modules"
   "${CMAKE_SOURCE_DIR}/../../cmake/modules")
 
+include(LanguageVersions)
 include(SwiftProjectVersion)
 project(SwiftObservation
   LANGUAGES Swift CXX
@@ -22,9 +23,6 @@ if(NOT PROJECT_IS_TOP_LEVEL)
   message(SEND_ERROR "Swift Observation must build as a standalone project")
 endif()
 
-set(CMAKE_CXX_STANDARD 17)
-set(CMAKE_CXX_STANDARD_REQUIRED YES)
-set(CMAKE_CXX_EXTENSIONS NO)
 set(CMAKE_POSITION_INDEPENDENT_CODE YES)
 
 set(${PROJECT_NAME}_SWIFTC_SOURCE_DIR

Runtimes/Supplemental/StringProcessing/CMakeLists.txt

@@ -14,6 +14,7 @@ list(APPEND CMAKE_MODULE_PATH
   "${CMAKE_SOURCE_DIR}/../cmake/modules"
   "${CMAKE_SOURCE_DIR}/../../cmake/modules")
 
+include(LanguageVersions)
 include(SwiftProjectVersion)
 project(SwiftStringProcessing
   LANGUAGES Swift C

Runtimes/Supplemental/Synchronization/CMakeLists.txt

@@ -13,6 +13,7 @@ list(APPEND CMAKE_MODULE_PATH
   "${CMAKE_SOURCE_DIR}/../cmake/modules"
   "${CMAKE_SOURCE_DIR}/../../cmake/modules")
 
+include(LanguageVersions)
 include(SwiftProjectVersion)
 project(SwiftSynchronization
   LANGUAGES Swift

Runtimes/Supplemental/Volatile/CMakeLists.txt

@@ -13,6 +13,7 @@ list(APPEND CMAKE_MODULE_PATH
   "${CMAKE_SOURCE_DIR}/../cmake/modules"
   "${CMAKE_SOURCE_DIR}/../../cmake/modules")
 
+include(LanguageVersions)
 include(SwiftProjectVersion)
 project(SwiftVolatile
   LANGUAGES Swift

Runtimes/Supplemental/cmake/modules/LanguageVersions.cmake

@@ -0,0 +1,5 @@
+set(CMAKE_Swift_LANGUAGE_VERSION 5)
+
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED YES)
+set(CMAKE_CXX_EXTENSIONS NO)