Fully qualify (preprend ::) calls to math functions from libc

Summary:
This can cause a compile failure in cases like:

double log(double);
namespace foo {
  namespace log {}
}
using namespace foo;
void bar(int i) {
  log((double)i);
}

Reviewers: EricWF, mclow.lists

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D29804

llvm-svn: 294696