Fix an logic error in the clang driver preventing crtfastmath.o from linking when -Ofast is used without -ffast-math

In gcc using -Ofast forces linking of crtfastmath.o.
In the current clang crtfastmath.o is only linked when -ffast-math/-funsafe-math-optimizations passed. It can lead to performance issues, when using only -Ofast without explicit -ffast-math (I faced with it).
My patch fixes inconsistency with gcc behaviour and also introduces few tests on it.

Patch by Zinovy Nis!

Differential Revision: http://llvm-reviews.chandlerc.com/D3114

llvm-svn: 204742
diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index c318b60..5cc19f5 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -2049,3 +2049,7 @@
 
   return std::make_pair(IncludedFlagsBitmask, ExcludedFlagsBitmask);
 }
+
+bool clang::driver::isOptimizationLevelFast(const llvm::opt::ArgList &Args) {
+  return Args.hasFlag(options::OPT_Ofast, options::OPT_O_Group, false);
+}