Add ASTUnit::LoadFromCompilerInvocation, which does what it says.

Also, add an -ast-from-source option to index-test which allows index-test to
run on source files directly.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@90223 91177308-0d34-0410-b5e6-96231b3b80d8