[frontend] Add support for disabling the "inline" keyword using

[frontend] Add support for disabling the "inline" keyword using
-fno-inline-functions.

This behaves much like -fno-inline in gcc, but based on a discussion with
Daniel it was decided that -fno-inline-functions should subsume -fno-inline.
Please speak up if you object.  The -fno-inline flag remains ignored.
Final part of rdar://10972766

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@152754 91177308-0d34-0410-b5e6-96231b3b80d8