[build-windows-toolchain] Enable building on non-UTF-8 Windows machine

`cl` will assume source and execution to be in local charset, so
we should pass in `/utf-8` flags or prefer `clang-cl` instead.

Author: stevapple

utils/build-windows-toolchain.bat

@@ -85,9 +85,9 @@ cmake ^
 
   -D CMAKE_BUILD_TYPE=%CMAKE_BUILD_TYPE% ^
   -D CMAKE_C_COMPILER=cl ^
-  -D CMAKE_C_FLAGS="/GS- /Oy /Gw /Gy" ^
+  -D CMAKE_C_FLAGS="/GS- /Oy /Gw /Gy /utf-8" ^
   -D CMAKE_CXX_COMPILER=cl ^
-  -D CMAKE_CXX_FLAGS="/GS- /Oy /Gw /Gy" ^
+  -D CMAKE_CXX_FLAGS="/GS- /Oy /Gw /Gy /utf-8" ^
   -D CMAKE_MT=mt ^
   -D CMAKE_EXE_LINKER_FLAGS="/INCREMENTAL:NO" ^
   -D CMAKE_SHARED_LINKER_FLAGS="/INCREMENTAL:NO" ^
@@ -398,9 +398,9 @@ cmake ^
   -B %BuildRoot%\12 ^
 
   -D CMAKE_BUILD_TYPE=%CMAKE_BUILD_TYPE% ^
-  -D CMAKE_C_COMPILER=cl ^
+  -D CMAKE_C_COMPILER=%BuildRoot%/1/bin/clang-cl.exe ^
   -D CMAKE_C_FLAGS="/GS- /Oy /Gw /Gy" ^
-  -D CMAKE_CXX_COMPILER=cl ^
+  -D CMAKE_CXX_COMPILER=%BuildRoot%/1/bin/clang-cl.exe ^
   -D CMAKE_CXX_FLAGS="/GS- /Oy /Gw /Gy" ^
   -D CMAKE_MT=mt ^
   -D CMAKE_Swift_COMPILER=%BuildRoot%/1/bin/swiftc.exe ^