Use legacy mangling instead of v0.

`v0` mangling mangles generics, but can only handle const generic
arguments of simple types. As `Image!` types use const generic enums,
things break horribly and compiler panics.
`legacy` doesn't even attempt to mangle generics, which is probably
fine.

crates/spirv-builder/src/lib.rs

@@ -357,7 +357,7 @@ fn invoke_rustc(builder: &SpirvBuilder) -> Result<PathBuf, SpirvBuilderError> {
     };
 
     let rustflags = format!(
-        "-Z codegen-backend={} -Zsymbol-mangling-version=v0{}{}",
+        "-Z codegen-backend={} -Zsymbol-mangling-version=legacy{}{}",
         rustc_codegen_spirv.display(),
         feature_flag,
         llvm_args,