CMake: add ENABLE_RTTI option (#4382)

Fixes

 $ CXX=clang++ cmake -H. -Bout -DSPIRV_USE_SANITIZER=vptr
 $ make -C out
 ...
 clang: error: invalid argument '-fsanitize=vptr' not allowed with '-fno-rtti'

CMakeLists.txt

@@ -33,6 +33,7 @@ include(GNUInstallDirs)
 set(CMAKE_POSITION_INDEPENDENT_CODE ON)
 set(CMAKE_CXX_STANDARD 11)
 
+option(ENABLE_RTTI "Enables RTTI" OFF)
 option(SPIRV_ALLOW_TIMERS "Allow timers via clock_gettime on supported platforms" ON)
 
 if("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux")
@@ -178,11 +179,14 @@ function(spvtools_default_compile_options TARGET)
   target_compile_options(${TARGET} PRIVATE ${SPIRV_WARNINGS})
 
   if (${COMPILER_IS_LIKE_GNU})
-    target_compile_options(${TARGET} PRIVATE
-      -std=c++11 -fno-exceptions -fno-rtti)
+    target_compile_options(${TARGET} PRIVATE -std=c++11 -fno-exceptions)
     target_compile_options(${TARGET} PRIVATE
       -Wall -Wextra -Wno-long-long -Wshadow -Wundef -Wconversion
       -Wno-sign-conversion)
+
+    if(NOT ENABLE_RTTI)
+        add_compile_options(-fno-rtti)
+    endif()
     # For good call stacks in profiles, keep the frame pointers.
     if(NOT "${SPIRV_PERF}" STREQUAL "")
       target_compile_options(${TARGET} PRIVATE -fno-omit-frame-pointer)