[DEBUGINFO] Add support for emission of the debug directives only.

Summary:
Added option -gline-directives-only to support emission of the debug directives
only. It behaves very similar to -gline-tables-only, except that it sets
llvm debug info emission kind to
llvm::DICompileUnit::DebugDirectivesOnly.

Reviewers: echristo

Subscribers: aprantl, fedor.sergeev, JDevlieghere, cfe-commits

Differential Revision: https://reviews.llvm.org/D51177

llvm-svn: 341212