Define toString() as a generic function to get a string for error message.

We have different functions to stringize objects to construct
error messages. For InputFile, we have getFilename, and for
InputSection, we have getName. You had to memorize them.

I think this is the case where the function overloading comes in handy.

This patch defines toString() functions that are overloaded for all these
types, so that you just call it in error().

Differential Revision: https://reviews.llvm.org/D27030

llvm-svn: 287787