[Driver] Change the way we deal with local lldbinit files.

Currently we have special handling for local lldbinit files in the
driver. At the same time, we have an SB API named
`SourceInitFileInCurrentWorkingDirectory` that does the same thing.

This patch removes the special handling from the driver and uses the API
instead. In addition to the obvious advantages of having one canonical
way of doing things and removing code duplication, this change also
means that the code path is the same for global and local lldb init
files.

Differential revision: https://reviews.llvm.org/D61577

llvm-svn: 360077