[clang][cmake] Don't pass -fno-strict-aliasing for GCC

This was added a long time ago..
* to the Makefiles in 40fee63;
* first to CMake in b3ce035;
* then moved to only apply when building Clang with GCC in
  c5635a6.

This shouldn't be needed these days. If an issue does arise, it really
ought to be documented better and the cause will certainly be different
than it was back then.

The two GCC bugs cited in 40fee63 were:
* https://gcc.gnu.org/PR41874
* https://gcc.gnu.org/PR41838
and both are long-fixed. Not only that, if those issues did come up again,
we'd be better off doing -Wno-strict-aliasing where appropriate if there
weren't a real code issue or some suppression that was tighter in scope
wasn't appropriate.

Author: thesamesam

clang/CMakeLists.txt

@@ -345,9 +345,6 @@ configure_file(
 # Add appropriate flags for GCC
 if (LLVM_COMPILER_IS_GCC_COMPATIBLE)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-common -Woverloaded-virtual")
-  if (NOT "${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-strict-aliasing")
-  endif ()
 
   # Enable -pedantic for Clang even if it's not enabled for LLVM.
   if (NOT LLVM_ENABLE_PEDANTIC)