Add a new -mimplicit-float option for symmetry with -mno-implicit-float.

This new option is the default, but it is useful to have a flag to override
-mno-implicit-float by putting -mimplicit-float later on the command line.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179309 91177308-0d34-0410-b5e6-96231b3b80d8