build: avoid use of -external:I on Windows

Avoid the use of the new `-external:I` to indicate the system headers
for building the standard library on Windows as this will impact the
header search order and due to the compiler swapping and mixed build,
this breaks the runtime build on Windows.

Author: compnerd

stdlib/CMakeLists.txt

@@ -5,6 +5,15 @@
 # standard library from the Swift compiler build.
 project(swift-stdlib LANGUAGES C CXX)
 
+# CMake passes `-external:I` to clang-cl which results in the search order being
+# altered, and this impacts the definitions of the intrinsics. When building
+# with a MSVC toolset 19.29.30036.3 or newer, this will prevent the runtime from
+# being built on Windows.  Since we know that we only support `clang-cl` as the
+# compiler for the runtime due to the use of the Swift calling convention, we
+# simply override the CMake behaviour unconditionally.
+set(CMAKE_INCLUDE_SYSTEM_FLAG_C "-I")
+set(CMAKE_INCLUDE_SYSTEM_FLAG_CXX "-I")
+
 # Add path for custom CMake modules.
 list(APPEND CMAKE_MODULE_PATH
   "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules")