Add ASTUnit::LoadFromCommandLine, which creates an ASTUnit out of a list of

(clang/driver) command line arguments (including the source file).
 - The arguments are expected to include the source file.

 - The idea is that even though this is a somewhat odd API, its the form which
   many tools can most easily use (for example, by interposing with the compiler).

Also, switch index-test's -ast-from-source to use this entry point, and provide
a -arg command line argument which can be used to test that the command line
arguments are handled correctly.

llvm-svn: 90288