[Driver] Add support for -finline-functions and /Ob2 flags

Description

[Driver] Add support for -finline-functions and /Ob2 flags

-finline-functions and /Ob2 are currently ignored by Clang. The only way to
enable inlining is to use the global O flags, which also enable other options,
or to emit LLVM bitcode using Clang, then running opt by hand with the inline
pass.

This patch allows to simply use the -finline-functions flag (same as GCC) or
/Ob2 in clang-cl mode to enable inlining without other optimizations.

This is the first patch of a serie to improve support for the /Ob flags.

Patch by Rudy Pons <rudy.pons@ilod.org>!

Differential Revision: http://reviews.llvm.org/D20576

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