MC: Add support for .cfi_startproc simple

This commit allows LLVM MC to process .cfi_startproc directives when
they are followed by an additional `simple' identifier. This signals to
elide the emission of target specific CFI instructions that would
normally occur initially.

This fixes PR16587.

Differential Revision: http://llvm-reviews.chandlerc.com/D2624


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200227 91177308-0d34-0410-b5e6-96231b3b80d8