Partially revert r186903.

It turns out that Plum Hall depends on us not emitting an error on
integer literals which fit into long long, but fit into
unsigned long long.  So C99 conformance requires not conforming to C99. :)

llvm-svn: 187172
diff --git a/clang/lib/Lex/PPExpressions.cpp b/clang/lib/Lex/PPExpressions.cpp
index 5cba35b..87c0a6a 100644
--- a/clang/lib/Lex/PPExpressions.cpp
+++ b/clang/lib/Lex/PPExpressions.cpp
@@ -259,7 +259,7 @@
       if (!Literal.isUnsigned && Result.Val.isNegative()) {
         // Don't warn for a hex or octal literal: 0x8000..0 shouldn't warn.
         if (ValueLive && Literal.getRadix() == 10)
-          PP.Diag(PeekTok, diag::err_integer_too_large_for_signed);
+          PP.Diag(PeekTok, diag::warn_integer_too_large_for_signed);
         Result.Val.setIsUnsigned(true);
       }
     }