add support for -fno-math-errno, and validate that it affects sema properly.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@64708 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/Driver/clang.cpp b/Driver/clang.cpp
index a2578a4..deea08c 100644
--- a/Driver/clang.cpp
+++ b/Driver/clang.cpp
@@ -219,7 +219,7 @@
 static llvm::cl::opt<bool>
 MathErrno("fmath-errno", 
           llvm::cl::desc("Require math functions to respect errno"),
-          llvm::cl::init(true));
+          llvm::cl::init(true), llvm::cl::AllowInverse);
 
 //===----------------------------------------------------------------------===//
 // Analyzer Options.