[Driver] Unbreak hexagon build bots

AddHexagonTargetArgs didn't respect the driver flags by unconditionally
pushing -fno-signed-char.  Instead, add Hexagon handling to
isSignedCharDefault.

llvm-svn: 238106

clang/lib/Driver/Tools.cpp

@@ -497,6 +497,7 @@ static bool isSignedCharDefault(const llvm::Triple &Triple) {
       return true;
     return false;
 
+  case llvm::Triple::hexagon:
   case llvm::Triple::ppc64le:
   case llvm::Triple::systemz:
   case llvm::Triple::xcore:
@@ -1746,7 +1747,6 @@ void Clang::AddX86TargetArgs(const ArgList &Args,
 
 void Clang::AddHexagonTargetArgs(const ArgList &Args,
                                  ArgStringList &CmdArgs) const {
-  CmdArgs.push_back("-fno-signed-char");
   CmdArgs.push_back("-mqdsp6-compat");
   CmdArgs.push_back("-Wreturn-type");