add the -mrecip driver flag and process its options (2nd try)

The first try to land this (r238055) was reverted due to bot failures
caused by the LLVM part of the patch. That was hopefully fixed by r238788,
and the LLVM patch was resubmitted at r238842.

This is the front-end counterpart to D8982.

The -mrecip option interface is based on maintaining compatibility with gcc:
https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gcc/i386-and-x86-64-Options.html#index-mrecip_003dopt-1627
https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gcc/RS_002f6000-and-PowerPC-Options.html#index-mrecip-2289

...while adding more functionality (allowing users to specify the number of refinement steps for each
estimate type).

Differential Revision: http://reviews.llvm.org/D8989

llvm-svn: 238851