Update comments

Author: jsji

llvm/unittests/CMakeLists.txt

@@ -14,7 +14,16 @@ function(add_llvm_target_unittest test_dir_name)
   add_llvm_unittest(${test_dir_name} DISABLE_LLVM_LINK_LLVM_DYLIB ${ARGN})
 endfunction()
 
-# GCC may emit false positive warnings against LLVM's style guide.
+# gtest macros like EXPECT_TRUE are expanded to a single line
+# multi-statement code with if/else. eg:
+#   if (...)
+#     EXPECT_TURE(...)
+# will be expanded into something like:
+#   if(...)
+#     switch (0) case 0: default: if (...) ; else return;;
+# GCC may emit false positive dangling-else warnings for such code.
+# However, such warnings are actually against LLVM's style guide.
+# disable the warning for GCC so that one can enbable Werror.
 if (CMAKE_COMPILER_IS_GNUCXX)
   list(APPEND LLVM_COMPILE_FLAGS "-Wno-dangling-else")
 endif ()