[build] Disable checked iterators on Windows

Trying to run a debug build of the Swift compiler with a host Swift toolchain on Windows was failing with a linker error due to a mismatched `_ITERATOR_DEBUG_LEVEL` macro.

This avoids the linker error by disabling bounds-checked iterators on Windows. See the inline comment for details.

Author: egorzhdan

CMakeLists.txt

@@ -419,6 +419,17 @@ set(SWIFT_STDLIB_MSVC_RUNTIME_LIBRARY
   ${SWIFT_STDLIB_MSVC_RUNTIME_LIBRARY_default}
   CACHE STRING "MSVC Runtime Library for the standard library")
 
+if("${CMAKE_SYSTEM_NAME}" STREQUAL "Windows" AND BOOTSTRAPPING_MODE STREQUAL "HOSTTOOLS" AND
+    CMAKE_BUILD_TYPE STREQUAL "Debug")
+  # Building with the host Swift toolchain requires linking just-built binaries
+  # against the host Swift runtime. In debug builds, that means linking a debug
+  # binary against a release binary. The MSVC linker does not normally permit
+  # this, since debug builds enable bounds-checked C++ iterators by default,
+  # which are not ABI-compatible with regular iterators. Let's instruct MSVC to
+  # disable bounds-checked iterators to make it possible to do a debug build of
+  # the Swift compiler with a host toolchain.
+  add_definitions(-D_ITERATOR_DEBUG_LEVEL=0)
+endif()
 
 if(BRIDGING_MODE STREQUAL "DEFAULT" OR NOT BRIDGING_MODE)
   if(CMAKE_BUILD_TYPE STREQUAL "Debug" OR "${SWIFT_HOST_VARIANT_SDK}" STREQUAL "WINDOWS" OR (CMAKE_Swift_COMPILER AND CMAKE_Swift_COMPILER_VERSION VERSION_LESS 5.8))