build: enable building in Debug mode on Windows

Windows does not have an ABI stable C/C++ runtime, and is ABI
incompatible between debug and release mode on the same version even.
Ensure that we always use the release mode DLL variant of the C/C++
runtime (`/MD`) allowing us to build the client code in debug or release
mode.

Author: compnerd

CMakeLists.txt

@@ -6,6 +6,10 @@
 # See http://swift.org/LICENSE.txt for license information
 # See http://swift.org/CONTRIBUTORS.txt for Swift project authors
 
+if(POLICY CMP0091)
+  cmake_policy(SET CMP0091 NEW)
+endif()
+
 cmake_minimum_required(VERSION 3.19.6)
 
 list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules)
@@ -14,12 +18,14 @@ project(SwiftDriver LANGUAGES C Swift)
 
 set(CMAKE_Swift_LANGUAGE_VERSION 5)
 set(CMAKE_Swift_MODULE_DIRECTORY ${CMAKE_BINARY_DIR}/swift)
+set(CMAKE_Swift_COMPILE_OPTIONS_MSVC_RUNTIME_LIBRARY MultiThreadedDLL)
 
 set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
 set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
 
 set(CMAKE_MACOSX_RPATH YES)
+set(CMAKE_MSVC_RUNTIME_LIBRARY MultiThreadedDLL)
 
 # ensure Swift compiler can find _CSwiftScan
 add_compile_options($<$<COMPILE_LANGUAGE:Swift>:-I$<SEMICOLON>${CMAKE_CURRENT_SOURCE_DIR}/Sources/CSwiftScan/include>)