(this is clang related part)
Without these explicit includes, removing other headers, who implicitly
include llvm-config.h, may have non-trivial side effects. For example,
`clagd` may report even `llvm-config.h` as "no used" in case it defines
a macro, that is explicitly used with #ifdef. It is actually amplified
with different build configs which use different set of macros.
The forwarding header is left in place because of its use in
`polly/lib/External/isl/interface/extract_interface.cc`, but I have
added a GCC warning about the fact it is deprecated, because it is used
in `isl` from where it is included by Polly.
Make it possible to disable building the decision forest ranking
model for clangd. To unbreak build of Clangd on PPC32 in gentoo, see
https://bugs.gentoo.org/829602
Based on D138520.
Reviewed By: sammccall
Differential Revision: https://reviews.llvm.org/D139107
