Without this patch, Clacc source-to-source mode when requested by
`-fopenacc-print` (but not `-fopenacc-ast-print`) often fails asserts
or mangles text when certain parts of OpenACC constructs expand from
preprocessor macros or when the `_Pragma` directive form is used.
With this patch, Clacc reports error diagnostics for cases not yet
handled, and other cases are fixed.

Such an error diagnostic is reported when either the first token is
expanded from a macro (thus the `_Pragma` form is used) or the
associated statement must be rewritten but its last token is expanded
from a macro.

This patch also redesigns `ACCExecutableDirective::getConstructRange`
to support the above changes as well as upcoming changes that will
provide source location information via the OpenACC Profiling
Interface.