[libclang] Add entry points that take a full command line including argv[0].

This provides both a more uniform interface and makes libclang behave like
clang tooling wrt relative paths against argv[0]. This is necessary for
finding paths to a c++ standard library relative to a clang binary given
in a compilation database. It can also be used to find paths relative to
libclang.so if the full path to it is passed in.

Differential Revision: http://reviews.llvm.org/D14695

llvm-svn: 253466