add the -mrecip driver flag and process its options (3rd try)

The 1st and 2nd tries to land this (r238055, r238851) were reverted due to
bot failures caused by the LLVM part of the patch. That was hopefully fixed 
after r239001.

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



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@239536 91177308-0d34-0410-b5e6-96231b3b80d8