build: improve the builds with Visual Studio

Visual Studio's compiler does not accept `-Werror=switch`.  Use the equivalent
`-we4062` option instead.  Avoid using the `--` separate that clang uses to
identify that the options that follow are files and not options on the Visual
Studio compiler to reduce the unnecessary spew when building on Windows.

Author: compnerd

cmake/modules/ClangClCompileRules.cmake

@@ -1,8 +1,10 @@
 
 # clang-cl interprets paths starting with /U as macro undefines, so we need to
 # put a -- before the input file path to force it to be treated as a path.
-string(REPLACE "-c <SOURCE>" "-c -- <SOURCE>" CMAKE_C_COMPILE_OBJECT "${CMAKE_C_COMPILE_OBJECT}")
-string(REPLACE "-c <SOURCE>" "-c -- <SOURCE>" CMAKE_CXX_COMPILE_OBJECT "${CMAKE_CXX_COMPILE_OBJECT}")
+if(NOT MSVC AND "${CMAKE_SIMULATE_ID}" STREQUAL "MSVC")
+  string(REPLACE "-c <SOURCE>" "-c -- <SOURCE>" CMAKE_C_COMPILE_OBJECT "${CMAKE_C_COMPILE_OBJECT}")
+  string(REPLACE "-c <SOURCE>" "-c -- <SOURCE>" CMAKE_CXX_COMPILE_OBJECT "${CMAKE_CXX_COMPILE_OBJECT}")
+endif()
 
 # NOTE(compnerd) incremental linking is known to cause corruption in the
 # protocol conformance tables.  Avoid using incremental links with Visual

cmake/modules/SwiftSharedCMakeConfig.cmake

@@ -270,7 +270,11 @@ endmacro()
 macro(swift_common_cxx_warnings)
   # Make unhandled switch cases be an error in assert builds
   if(DEFINED LLVM_ENABLE_ASSERTIONS)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror=switch")
+    check_cxx_compiler_flag("-Werror=switch" CXX_SUPPORTS_WERROR_SWITCH_FLAG)
+    append_if(CXX_SUPPORTS_WERROR_SWITCH_FLAG "-Werror=switch" CMAKE_CXX_FLAGS)
+
+    check_cxx_compiler_flag("/we4062" CXX_SUPPORTS_WE4062)
+    append_if(CXX_SUPPORTS_WE4062 "/we4062" CMAKE_CXX_FLAGS)
   endif()
 
   check_cxx_compiler_flag("-Werror -Wdocumentation" CXX_SUPPORTS_DOCUMENTATION_FLAG)