Compile clang with `-Wall -Wshadow -Werror`

It was not compiling any of the code with warnings prior to this.
Signed-off-by: Enji Cooper <yaneurabeya@gmail.com>

googletest/cmake/internal_utils.cmake

@@ -77,6 +77,10 @@ macro(config_compiler_and_linker)
     # Suppress "unreachable code" warning
     # http://stackoverflow.com/questions/3232669 explains the issue.
     set(cxx_base_flags "${cxx_base_flags} -wd4702")
+  elseif (CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+    set(cxx_base_flags "-Wall -Wshadow -Werror")
+    set(cxx_exception_flags "-fexceptions")
+    set(cxx_no_exception_flags "-fno-exceptions")
   elseif (CMAKE_COMPILER_IS_GNUCXX)
     set(cxx_base_flags "-Wall -Wshadow -Werror")
     if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 7.0.0)