Summary:
See comments in patch; we were assuming that some stdlib math functions
would be defined in namespace std, when in fact the spec says they
should be defined in the global namespace.  libstdc++4.9 became more
conforming and broke us.

This new implementation seems to cover the known knowns.

Reviewers: rsmith

Subscribers: cfe-commits, tra

Differential Revision: http://reviews.llvm.org/D18882

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@265751 91177308-0d34-0410-b5e6-96231b3b80d8