[DWARF parser] Teach DIContext to fetch short (non-linkage) function names for a given address.

Change --functions option in llvm-symbolizer tool to accept
values "none", "short" or "linkage". Update the tests and docs
accordingly.

llvm-svn: 209050
diff --git a/llvm/docs/CommandGuide/llvm-symbolizer.rst b/llvm/docs/CommandGuide/llvm-symbolizer.rst
index dfbdb3a..ce2d9c0 100644
--- a/llvm/docs/CommandGuide/llvm-symbolizer.rst
+++ b/llvm/docs/CommandGuide/llvm-symbolizer.rst
@@ -61,11 +61,14 @@
 -------
 
 .. option:: -obj
+
   Path to object file to be symbolized.
 
-.. option:: -functions
+.. option:: -functions=[none|short|linkage]
 
-  Print function names as well as source file/line locations. Defaults to true.
+  Specify the way function names are printed (omit function name,
+  print short function name, or print full linkage name, respectively).
+  Defaults to ``linkage``.
 
 .. option:: -use-symbol-table