Introduce -fembed-bitcode driver option

Summary:
This is the clang driver part of the change to embedded bitcode. This
includes:
1. -fembed-bitcode option which breaks down the compilation into two
stages. The first stage emits optimized bitcode and the second stage
compiles bitcode into object file.
2. -fembed-bitcode-marker option which doesn't really break down to
two stages to speedup the compilation flow.
3. pass the correct linker flag to darwin linker if tool chains supports
embedded bitcode.

Reviewers: rsmith, thakis

Subscribers: thakis, cfe-commits

Differential Revision: http://reviews.llvm.org/D17390

llvm-svn: 262282