Revert commit eb1a9be4b6 (XL: Recognize compilers identified by
__ibmxl__, 2018-03-05, v3.11.0-rc3~4^2). It is no longer needed because
we now use `__ibmxl__` to identify with compiler id `XLClang`.
IBM XL C/C++ for Linux versions 13.1.6 and above no longer define
`__IBMC__` or `__IBMCPP__` by default (see `-qxlcompatmacros`).
Instead `__ibmxl__` now identifies the compiler along with some
related new version macros.
Fixes: #17784
This will allow sharing of the logic of the order to test compilers in
and the preprocessor macros used to do that and to determine the
version components.