build: emulate /GR- when compiling for Windows

The /GR- emulation is needed to support building on Windows.
std::function in Microsoft's C++ runtime requires the use of `typeid`.
We do not want to emit RTTI data.  `-frtti -fno-rtti-data` allows the
use of the `typeid` and `dynamic_cast` behaviours but will not emit the
RTTI data.  With this change, it is possible to build the Windows
runtime once again.

Author: compnerd

cmake/modules/AddSwift.cmake

@@ -248,6 +248,13 @@ function(_add_variant_c_compile_flags)
     list(APPEND result "-D_DLL")
     # NOTE: We assume that we are using VS 2015 U2+
     list(APPEND result "-D_ENABLE_ATOMIC_ALIGNMENT_FIX")
+
+    # msvcprt's std::function requires RTTI, but we do not want RTTI data.
+    # Emulate /GR-
+    if(NOT SWIFT_COMPILER_IS_MSVC_LIKE)
+      list(APPEND result -frtti)
+      list(APPEND result -Xclang;-fno-rtti-data)
+    endif()
   endif()
 
   if(CFLAGS_ENABLE_ASSERTIONS)