[libcxx] Disable failing test for no-exceptions build

[ostannard]

Summary

This test tries to create a 2 GiB std::string, catching the bad_alloc exception if the allocation fails. However, for no-exceptions builds there is no way for the error to be reported, so this crashes with a null pointer dereference.

[ldionne]

Please add a comment explaining why it's disabled for no-exceptions, and commit. It's far from obvious.

libcxx/test/std/input.output/stream.buffers/streambuf/streambuf.protected/streambuf.put.area/pbump2gig.pass.cpp

 
 // template <class charT, class traits = char_traits<charT> >
 // class basic_streambuf;
 
 // void pbump(int n);
 //
 // REQUIRES: long_tests
 
+// Unsupported for no-exceptions builds because they have no way to report an
+// allocation failure when attempting to allocate the 2GiB string.
+// UNSUPPORTED: no-exceptions
+
 #include <sstream>
 #include <cassert>
 #include "test_macros.h"
 
 struct SB : std::stringbuf
 {
   SB() : std::stringbuf(std::ios::ate|std::ios::out) { }
   const char* pubpbase() const { return pbase(); }
   const char* pubpptr() const { return pptr(); }
 };
 
 int main(int, char**)
 {
-#ifndef TEST_HAS_NO_EXCEPTIONS
     try {
-#endif
         std::string str(2147483648, 'a');
         SB sb;
         sb.str(str);
         assert(sb.pubpbase() <= sb.pubpptr());
-#ifndef TEST_HAS_NO_EXCEPTIONS
     }
     catch (const std::length_error &) {} // maybe the string can't take 2GB
     catch (const std::bad_alloc    &) {} // maybe we don't have enough RAM
-#endif
 
   return 0;
 }