Don't claim exception related arguments when looking at RTTIMode

[filcab]

Summary

We were claiming the -f*exceptions arguments when looking for the
RTTIMode. This makes us not warn about unused arguments if compiling a C
file with -fcxx-exceptions.

This patch fixes it by not claiming the exception-related arguments at
that point.

[filcab]

Fix the test by actually forcing the test's special case to treat it as a C file

[samsonov]

LGTM

lib/Driver/ToolChain.cpp

static ToolChain::RTTIMode CalculateRTTIMode(const ArgList &Args,
   }
 
   // -frtti is default, except for the PS4 CPU.
   if (!Triple.isPS4CPU())
     return ToolChain::RM_EnabledImplicitly;
 
   // On the PS4, turning on c++ exceptions turns on rtti.
   // We're assuming that, if we see -fexceptions, rtti gets turned on.
-  Arg *Exceptions = Args.getLastArg(
+  Arg *Exceptions = Args.getLastArgNoClaim(
       options::OPT_fcxx_exceptions, options::OPT_fno_cxx_exceptions,
       options::OPT_fexceptions, options::OPT_fno_exceptions);
   if (Exceptions &&
       (Exceptions->getOption().matches(options::OPT_fexceptions) ||
        Exceptions->getOption().matches(options::OPT_fcxx_exceptions)))
     return ToolChain::RM_EnabledImplicitly;
 
   return ToolChain::RM_DisabledImplicitly;

test/Driver/rtti-options.cpp

 // Check that we emit warnings/errors for different combinations of
 // exceptions, rtti, and vptr sanitizer flags when targetting the PS4.
 // No warnings/errors should be emitted for unknown, except if combining
 // the vptr sanitizer with -fno-rtti
 
+// Special case: -fcxx-exceptions in C code should warn about unused arguments
+// RUN: %clang -### -c -fcxx-exceptions %s 2>&1 | FileCheck -check-prefix=CHECK-UNUSED %s
+// CHECK-UNUSED: warning: argument unused during compilation: '-fcxx-exceptions'
+
 // RUN: %clang -### -c -fno-rtti -frtti %s 2>&1 | FileCheck -check-prefix=CHECK-RTTI %s
 // RUN: %clang -### -c -frtti -fno-rtti %s 2>&1 | FileCheck -check-prefix=CHECK-NO-RTTI %s
 
 // -fsanitize=vptr
 // Make sure we only error/warn once, when trying to enable vptr and
 // undefined and have -fno-rtti
 // RUN: %clang -### -c -fsanitize=undefined -fsanitize=vptr -fno-rtti %s 2>&1 | FileCheck -check-prefix=CHECK-SAN-ERROR -check-prefix=CHECK-OK %s