Emit error when calling/declaring functions with unavailable vectors.

On some architectures, vector types may have a different ABI when
relevant target features are enabled.

As discussed in https://github.com/rust-lang/lang-team/issues/235, this
turns out to very easily lead to unsound code.

This commit makes it an error to declare or call functions using those
vector types in a context in which the corresponding target features are
disabled, if using an ABI for which the difference is relevant.

compiler/rustc_monomorphize/src/errors.rs

@@ -92,3 +92,21 @@ pub(crate) struct StartNotFound;
 pub(crate) struct UnknownCguCollectionMode<'a> {
     pub mode: &'a str,
 }
+
+#[derive(LintDiagnostic)]
+#[diag(monomorphize_abi_error_disabled_vector_type_def)]
+#[help]
+pub(crate) struct AbiErrorDisabledVectorTypeDef<'a> {
+    #[label]
+    pub span: Span,
+    pub required_feature: &'a str,
+}
+
+#[derive(LintDiagnostic)]
+#[diag(monomorphize_abi_error_disabled_vector_type_call)]
+#[help]
+pub(crate) struct AbiErrorDisabledVectorTypeCall<'a> {
+    #[label]
+    pub span: Span,
+    pub required_feature: &'a str,
+}