diff --git a/llvm/cmake/modules/AddLLVM.cmake b/llvm/cmake/modules/AddLLVM.cmake
--- a/llvm/cmake/modules/AddLLVM.cmake
+++ b/llvm/cmake/modules/AddLLVM.cmake
@@ -1589,6 +1589,10 @@
     list(APPEND LLVM_COMPILE_FLAGS "-Wno-gnu-zero-variadic-macro-arguments")
   endif()
 
+  if(MSVC)
+    list(APPEND LLVM_COMPILE_FLAGS "/execution-charset:utf-8")
+  endif()
+
   if (NOT DEFINED LLVM_REQUIRES_RTTI)
     set(LLVM_REQUIRES_RTTI OFF)
   endif()
diff --git a/llvm/cmake/modules/HandleLLVMOptions.cmake b/llvm/cmake/modules/HandleLLVMOptions.cmake
--- a/llvm/cmake/modules/HandleLLVMOptions.cmake
+++ b/llvm/cmake/modules/HandleLLVMOptions.cmake
@@ -489,6 +489,9 @@
     _UNICODE
   )
 
+  # Tell MSVC the LLVM sources are UTF-8 encoded.
+  append("/source-charset:utf-8" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
+
   if (LLVM_WINSYSROOT)
     if (NOT CLANG_CL)
       message(ERROR "LLVM_WINSYSROOT requires clang-cl")