LLVM-15+ dependency needed

[dabrahams]

Per this section, I'm opening an issue for the addition of LLVM-15 as a dependency to your Linux build image.

I'm not sure if that's really the right answer; if we should be adding something to our SPI manifest instead, please let us know. Note that our setup requires the generation of a pkgconfig file for LLVM. Or perhaps we should be using our own devcontainer. Please let us know!

/cc @kyouko-taiga

Local testing yields:

docker run --rm -v "$PWD":/host -w /host registry.gitlab.com/finestructure/spi-images:basic-5.8-latest swift build
WARNING: The requested image's platform (linux/amd64) does not match the detected host platform (linux/arm64/v8) and no specific platform was requested
warning: couldn't find pc file for llvm
[0/1] Planning build
[1/1] Compiling plugin Format Source Code
[2/2] Compiling plugin GenerateManual
[3/3] Compiling plugin Lint Source Code
[4/4] Compiling plugin TestGeneratorPlugin
[5/5] Compiling plugin ValidatorPlugin
Building for debugging...
/host/.build/checkouts/Swifty-LLVM/Sources/llvmshims/src/shim.cc:1:10: fatal error: 'llvm-c/TargetMachine.h' file not found
#include "llvm-c/TargetMachine.h"
         ^~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
[5/25] Compiling shim.cc

[dabrahams]

See #2545