Update to reflect changes in LLVM (issue #149).

"llvm/Support/system_error.h" was removed in LLVM r210803.  Replaced
llvm::error_code with std::error_code.

git-svn-id: http://include-what-you-use.googlecode.com/svn/trunk@557 812d1f90-f944-d88a-411a-122bdc93e1ea