Add compile error when both use-compiled-tools and use-installed-tools are enabled (EmbarkStudios#269)

* Add compile error when both use-compiled-tools and use-installed-tools are enabled

The compile error is located in the build.rs for rustc-codegen-spirv so that the error happens early, rather than after compiling spirv-tools.

* lint

* lint

Author: Hentropy

crates/rustc_codegen_spirv/build.rs

@@ -0,0 +1,8 @@
+// Putting this check here causes compilation failure seconds into the build,
+// putting it in lib.rs fails after minutes because spirv-tools gets compiled first.
+#[cfg(all(feature = "use-compiled-tools", feature = "use-installed-tools"))]
+compile_error!(
+    "Either \"use-compiled-tools\" (enabled by default) or \"use-installed-tools\" may be enabled."
+);
+
+fn main() {}