Introducing a plugin API and a simple HelloWorld Plugin example.
This patch adds the -load and -plugin flags to frontend driver and
the code around using custom frontend actions from within a plugin
shared library object.
It also adds to the Driver-help test to check the help option with the
updated driver flags.
Additionally, the patch creates a plugin-example test to check the
HelloWorld plugin example runs correctly. As part of this, a new CMake
flag (FLANG_BUILD_EXAMPLES) is added to allow the example to be built
and for the test to run.
This Plugin API has only been tested on Linux.