Added support for building against Android API-9 SDK

Created an abstraction for log2, llvm::Log2 in Support/MathExtras.h

Hid Android problems inside of it

Differential Revision: http://reviews.llvm.org/D9467

llvm-svn: 236680
diff --git a/llvm/lib/Analysis/ConstantFolding.cpp b/llvm/lib/Analysis/ConstantFolding.cpp
index a85e813..6a37f95 100644
--- a/llvm/lib/Analysis/ConstantFolding.cpp
+++ b/llvm/lib/Analysis/ConstantFolding.cpp
@@ -1438,7 +1438,7 @@
         case Intrinsic::fabs:
           return ConstantFoldFP(fabs, V, Ty);
         case Intrinsic::log2:
-          return ConstantFoldFP(log2, V, Ty);
+          return ConstantFoldFP(Log2, V, Ty);
         case Intrinsic::log:
           return ConstantFoldFP(log, V, Ty);
         case Intrinsic::log10: