Try a workaround suggested by @compnerd

Sources/CMakeLists.txt

@@ -36,11 +36,21 @@ target_link_libraries(llvmshims PRIVATE llvmc)
 # The Ubuntu GitHub runner demands this option: /usr/bin/ld.gold: error:
 # lib/libllvmshims.a(shim.cc.o): requires dynamic R_X86_64_PC32 reloc against
 # '_ZN4llvm17OptimizationLevel2O0E' which may overflow at runtime; recompile with -fPIC.  We don't
-# use -fPIC directly because it clang on Windows rejects it.
+# use -fPIC directly because clang on Windows rejects it.
 set_target_properties(llvmshims
   PROPERTIES
   POSITION_INDEPENDENT_CODE True)
 
+if(WIN32)
+  # According to compnerd, “a Microsoft update requires a newer clang
+  # than what swift ships. That’s a second chance that they pushed
+  # that breaks after working around this. The best option is to use a
+  # 6.0 pre release.”  We don't want to impose that on downstream
+  # clients, so we use this workaround, which isn't compatible with
+  # Swift/C++ interop.
+  target_compile_definitions(llvmshims PRIVATE _ALLOW_COMPILER_AND_STL_VERSION_MISMATCH)
+endif()
+
 #
 # The Swift module our clients depend on.
 #