Add --suppress-system-warnings (on by default, use =0 to disable)

Add --suppress-system-warnings (on by default, use =0 to disable)
 - For investigating warnings in system headers / builtins.
 - Currently also enables the behavior that allows silent redefinition
   of types in system headers. Conceptually these are separate but I
   didn't feel it was worth two options (or changing LangOptions).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@56163 91177308-0d34-0410-b5e6-96231b3b80d8