[patch] Make the llvm mangler depend only on DataLayout.

[Rafael Espíndola]

> This patch looks like a great improvement and a big cleanup!
>
> What does this mean for auto-upgrading of bitcode files?  Will old files continue to be readable with new tools?

Tools using lib/Target will continue to mangle as before. The target
creates a new DataLayout instead of using one in the bitcode file.
That is why none of the llc tests had to be updated for example.

Most tools don't use the mangler at all. For example, opt operates
only on llvm names, not the mangled names.

The only remaining tools that should not use lib/Target but should be
able to mangle are llvm-nm and llvm-ar. But using the final symbol
names in these tools will be a new feature, so it is probably ok if an
identity mangling is used for old bitcode files.

Cheers,
Rafael