Fix pretty printing the unspecified param of a variadic function

Summary:
 - Fix a bug in PrettyBuiltinDumper that returns "void" as the name for
  an unspecified builtin type. Since the unspecified param of a variadic
  function is considered a builtin of unspecified type in PDBs, we set
  "..." for its name.

  - Provide a method to determine if a PDBSymbolFunc is variadic in
  PrettyFunctionDumper since PDBSymbolFunc::getArgument() doesn't return the
  last unspecified-type param.

  - Add a pretty-func-dumper.test to test pretty dumping of variadic
  functions.

Reviewers: zturner, llvm-commits

Reviewed By: zturner

Differential Revision: https://reviews.llvm.org/D41801

llvm-svn: 322608