[CMake] Move RTTI flag calculation to AddLLVM.cmake (#175553)

This should help users who use AddLLVM.cmake without HandleLLVMOptions.
Note that remains MSVC is unlikely to work.

Follow up of #174084.

Author: aengelke

llvm/cmake/modules/AddLLVM.cmake

@@ -19,6 +19,18 @@ function(get_subproject_title outvar)
   endif ()
 endfunction(get_subproject_title)
 
+# Determine required flags to enable/disable RTTI.
+set(LLVM_CXXFLAGS_RTTI_DISABLE "")
+set(LLVM_CXXFLAGS_RTTI_ENABLE "")
+if(LLVM_COMPILER_IS_GCC_COMPATIBLE)
+  set(LLVM_CXXFLAGS_RTTI_DISABLE "-fno-rtti")
+elseif(MSVC)
+  set(LLVM_CXXFLAGS_RTTI_DISABLE "/GR-")
+  set(LLVM_CXXFLAGS_RTTI_ENABLE "/GR")
+elseif(CMAKE_CXX_COMPILER_ID MATCHES "XL")
+  set(LLVM_CXXFLAGS_RTTI_DISABLE "-qnortti")
+endif()
+
 function(llvm_update_compile_flags name)
   set(LLVM_COMPILE_CXXFLAGS "")
 

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -1162,22 +1162,13 @@ elseif(MSVC)
   endif()
 endif()
 
-# llvm_update_compile_flags adds one of these to each target.
-set(LLVM_CXXFLAGS_RTTI_DISABLE "")
-set(LLVM_CXXFLAGS_RTTI_ENABLE "")
-if(LLVM_COMPILER_IS_GCC_COMPATIBLE)
-  set(LLVM_CXXFLAGS_RTTI_DISABLE "-fno-rtti")
-elseif(MSVC)
+if(MSVC)
   # Remove flags here, for exceptions and RTTI.
   # Each target property or source property should be responsible to control
-  # them.
-  # CL.EXE complains to override flags like "/GR /GR-".
+  # them (see llvm_update_compile_flags).
+  # CL.EXE complains to override flags like "/GR /GR-". CMake adds them by default.
   string(REGEX REPLACE "(^| ) */EH[-cs]+ *( |$)" "\\1 \\2" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
   string(REGEX REPLACE "(^| ) */GR-? *( |$)" "\\1 \\2" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
-  set(LLVM_CXXFLAGS_RTTI_DISABLE "/GR-")
-  set(LLVM_CXXFLAGS_RTTI_ENABLE "/GR")
-elseif(CMAKE_CXXCOMPILER_ID MATCHES "XL")
-  set(LLVM_CXXFLAGS_RTTI_DISABLE "-qnortti")
 endif()
 
 # Provide public options to globally control RTTI and EH