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.
Reviewed By: awarzynski
Differential Revision: https://reviews.llvm.org/D106137
As `external-hello-world` is not really a test, I am moving it from
`flang/unittest/Runtime` to `flang/examples` (it makes a lot of sense as
an example). I've not modified the source code (apart from adjusting the
include paths).
Differential Revision: https://reviews.llvm.org/D104320