* [llvm] Update port to 15.0.1
The patchset is rebased onto llvmorg-15.0.1, with some significant
differences especially around install paths. The NixOS project
appears to be midway through a large change to the install path
mechanics, for the same purpose as this patchset.
When that has landed, it looks like most of these patches can go away.
* [llvm] x-add-version for 15.0.1
* [llvm] Add enable-ios feature flag
This passes straight through to COMPILER_RT_ENABLE_IOS, which is
defaulted to ON in LLVM. This is a problem if your build host
doesn't have the iOS SDK installed, such as on vcpkg CI.
Co-authored-by: Josh Channings <josh@channings.me.uk>
