Add the llvm-diff tool, which performs a relatively naive structural

Add the llvm-diff tool, which performs a relatively naive structural
diff of a function.  There's a lot of cruft in the current version, and
it's pretty far from perfect, but it's usable.

Currently only capable of comparing functions.  Currently ignores metadata.
Currently ignores most attributes of functions and instructions.

Patches welcome.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@109739 91177308-0d34-0410-b5e6-96231b3b80d8