[cmake] Fix LLVM_USE_SPLIT_DWARF

[hliao]

Summary

• Add -gsplit-dwarf as an option instead of a definition.
• Only add that option on compilers supporting dwarf splitting, such as clang and gcc.

[aprantl]

Not a big deal, but in LLDB we are compiling a handful of Objective-C++ (.mm) files. Would these get included by $<COMPILE_LANGUAGE:CXX>?

[hliao]

Apply to objc and objcxx.

[hliao]

Think again. There's no need to tell the languages for adding that compile option.

[aprantl]

One thing I ran into by accident recently was that -DLLVM_USE_SPLIT_DWARF=1 in a Release (no debug info) also turned non debug info. Is that still the case and if yes, should we conditionalize this on building with debug info?

[aprantl]

(sorry, that's more a question, and outside of the scope of this patch)

[hliao]

That makes sense to me. At least, in release build, we won't generate
unnecessary files.

[aprantl]

Thanks!

llvm/cmake/modules/HandleLLVMOptions.cmake

if (LLVM_USE_SANITIZER MATCHES ".*Undefined.*")
     set(BLACKLIST_FILE "${CMAKE_SOURCE_DIR}/utils/sanitizers/ubsan_blacklist.txt")
     if (EXISTS "${BLACKLIST_FILE}")
       append("-fsanitize-blacklist=${BLACKLIST_FILE}"
              CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
     endif()
   endif()
 endif()
 
-# Turn on -gsplit-dwarf if requested
-if(LLVM_USE_SPLIT_DWARF)
-  add_definitions("-gsplit-dwarf")
+# Turn on -gsplit-dwarf if requested in debug builds.
+if (LLVM_USE_SPLIT_DWARF AND
+    ((uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG") OR
+     (uppercase_CMAKE_BUILD_TYPE STREQUAL "RELWITHDEBINFO")))
+  # Limit to clang and gcc so far. Add compilers supporting this option.
+  if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" OR
+      CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+    add_compile_options(-gsplit-dwarf)
+  endif()
 endif()
 
 add_definitions( -D__STDC_CONSTANT_MACROS )
 add_definitions( -D__STDC_FORMAT_MACROS )
 add_definitions( -D__STDC_LIMIT_MACROS )
 
 # clang and gcc don't default-print colored diagnostics when invoked from Ninja.
 if (UNIX AND