Rollup merge of #60981 - alexcrichton:update-compiler-builtins, r=cuviper

Rollup merge of #60981 - alexcrichton:update-compiler-builtins, 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.