Clang: Fall back to llvm-rc when rc is unavailable

This change modifies how CMAKE_RC_COMPILER is configured to improve
the out-of-box experience for developers using Clang on Windows.
The previous behavior was to require the user to explicitly specify
the resource compiler when CMake was called. The new behavior
is to automatically attempt to locate the MSVC rc binary and use that
if it's found. If rc is not available, CMake will now fall back to
Clang's llvm-rc binary.

With this change in place, trivial C/C++ programs can be generated
with Ninja and Clang on Windows without running into errors about
a missing resource compiler.

Fixes: #19318

Modules/Platform/Windows-Clang.cmake

@@ -99,6 +99,21 @@ if("x${CMAKE_C_SIMULATE_ID}" STREQUAL "xMSVC"
             "or clang-cl as both C and C++ compilers.")
   endif()
 
+  if(NOT CMAKE_RC_COMPILER_INIT)
+    # Check if rc is already in the path
+    # This may happen in cases where the user is already in a visual studio environment when CMake is invoked
+    find_program(__RC_COMPILER_PATH NAMES rc)
+
+    # Default to rc if it's available, otherwise fall back to llvm-rc
+    if(__RC_COMPILER_PATH)
+      set(CMAKE_RC_COMPILER_INIT rc)
+    else()
+      set(CMAKE_RC_COMPILER_INIT llvm-rc)
+    endif()
+
+    unset(__RC_COMPILER_PATH CACHE)
+  endif()
+
   if ( "x${CMAKE_CXX_COMPILER_FRONTEND_VARIANT}" STREQUAL "xMSVC" OR "x${CMAKE_C_COMPILER_FRONTEND_VARIANT}" STREQUAL "xMSVC" )
     include(Platform/Windows-MSVC)