Ignore -mieee-fp.

It looks like clang always produce code with the ieee comparisons, so it is
safe to ignore this flag (we still error on -mno-ieee-fp).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191912 91177308-0d34-0410-b5e6-96231b3b80d8