This commit bumps the `compiler-builtins` dependency to 0.1.15 which
expects to have the source for `compiler-rt` provided externally if the
`c` feature is enabled. This then plumbs through the necessary support
in the build system to ensure that if the `llvm-project` directory is
checked out and present that we enable the `c` feature of
`compiler-builtins` and compile in all the C intrinsics.

… r=Centril

Move arbitrary self types's tests into ui/self

rust-lang#60944 (comment)

r? @Centril

…ns, r=cuviper

Bump compiler-builtins to 0.1.15

This commit bumps the `compiler-builtins` dependency to 0.1.15 which
expects to have the source for `compiler-rt` provided externally if the
`c` feature is enabled. This then plumbs through the necessary support
in the build system to ensure that if the `llvm-project` directory is
checked out and present that we enable the `c` feature of
`compiler-builtins` and compile in all the C intrinsics.

Bump LLVM submodule to fix MSP430 AsmPrinter and assembler syntax mismatch.

Moving on to steps 9 and 10 of the llvm bugfix [guide](https://rust-lang.github.io/rustc-guide/codegen/updating-llvm.html#bugfix-updates), now that Rust's copy of LLVM was [updated](rust-lang/llvm-project#13).

This PR closes issue rust-lang#59077. Nightlies following this PR should have working msp430 codegen again :D.

Thanks for the prompt response even though it took me a while to get this "simple" PR done!