llvm: simplify data layout check

Don't skip the inconsistent data layout check for custom LLVMs.

With #118708, all targets will have a simple test that would trigger this
check if LLVM's data layouts do change - so data layouts would be
corrected during the LLVM upgrade. Therefore, with builtin targets, this
check won't trigger with our LLVM because each target will have been
confirmed to work. With non-builtin targets, this check is probably
useful to have because you can change the data layout in your target and
if its wrong then that could lead to bugs.

When using a custom LLVM, the same justification makes sense for
non-builtin targets as with our LLVM, the user can update their target to
match their LLVM and that's probably a good thing to do. However, with
a custom LLVM, the user cannot change the builtin target data layouts if
they don't match - though given that the compiler's data layout is used
for layout computation and a bunch of other things - you could get some
bugs because of the mismatch and probably want to know about that.

`CFG_LLVM_ROOT` was also always set during local development with
`download-ci-llvm` so this bug would never trigger locally.

Signed-off-by: David Wood <david@davidtw.co>

compiler/rustc_codegen_llvm/src/errors.rs

@@ -244,3 +244,12 @@ pub(crate) struct CopyBitcode {
 pub struct UnknownCompression {
     pub algorithm: &'static str,
 }
+
+#[derive(Diagnostic)]
+#[diag(codegen_llvm_mismatch_data_layout)]
+pub struct MismatchedDataLayout<'a> {
+    pub rustc_target: &'a str,
+    pub rustc_layout: &'a str,
+    pub llvm_target: &'a str,
+    pub llvm_layout: &'a str,
+}