* Don't pass empty arguments to ExecuteAndWait because it can cause the

  sub-tool to start reading its standard input instead of the specified
  input.
* Clean up ouput of path names on error.
* Extend GetPathForLinkageItem to always search the LibraryPaths and thus
  make it suitable for an interface function (required by llvmc.cpp).
* Implement support for language-specific default library paths.

llvm-svn: 18188