[libclang] Expose some target information via the C API.

This allows users to query the target triple and target pointer width, which
would make me able to fix https://github.com/servo/rust-bindgen/issues/593 and
other related bugs in an elegant way (without having to manually parse the
target triple in the command line arguments).

Differential Revision: https://reviews.llvm.org/D32389

llvm-svn: 301648