[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
diff --git a/clang/lib/CodeGen/CGDebugInfo.cpp b/clang/lib/CodeGen/CGDebugInfo.cpp
index 02d716a..ac22dde 100644
--- a/clang/lib/CodeGen/CGDebugInfo.cpp
+++ b/clang/lib/CodeGen/CGDebugInfo.cpp
@@ -557,6 +557,9 @@
   case codegenoptions::DebugLineTablesOnly:
     EmissionKind = llvm::DICompileUnit::LineTablesOnly;
     break;
+  case codegenoptions::DebugDirectivesOnly:
+    EmissionKind = llvm::DICompileUnit::DebugDirectivesOnly;
+    break;
   case codegenoptions::LimitedDebugInfo:
   case codegenoptions::FullDebugInfo:
     EmissionKind = llvm::DICompileUnit::FullDebug;