Remove old fenv.h workaround for a historic clang driver bug

Tested and works fine with clang using libstdc++.

All indications are that this was fixed some time ago and isn't a problem with
any clang version we support.

I've added a note in PR6907 which is still open for some reason.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210485 91177308-0d34-0410-b5e6-96231b3b80d8
Added functions cross-reference test.
Originally this similar was initiated by Björn Steinbrink here:
http://reviews.llvm.org/D3437

Bug itself has been fixed by principal changes in MergeFunctions. Though
special checks for functions merging are still actual. And the test has
been accepted with slight modifications.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210486 91177308-0d34-0410-b5e6-96231b3b80d8
Look through addrspacecasts when turning ptr comparisons into
index comparisons.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210488 91177308-0d34-0410-b5e6-96231b3b80d8