Rollup merge of #135552 - amy-kwan:amy-kwan/reprc-struct-diagnostic-power-alignment, r=workingjubilee

[AIX] Lint on structs that have a different alignment in AIX's C ABI

This PR adds a linting diagnostic on AIX for repr(C) structs that are required to follow
the power alignment rule. A repr(C) struct needs to follow the power alignment rule if
the struct:
- Has a floating-point data type (greater than 4-bytes) as its first member, or
- The first member of the struct is an aggregate, whose recursively first member is a
   floating-point data type (greater than 4-bytes).

The power alignment rule for eligible structs is currently unimplemented, so a linting
diagnostic is produced when such a struct is encountered.

compiler/rustc_lint/src/lints.rs

@@ -1694,6 +1694,10 @@ pub(crate) struct OverflowingLiteral<'a> {
     pub lit: String,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(lint_uses_power_alignment)]
+pub(crate) struct UsesPowerAlignment;
+
 #[derive(LintDiagnostic)]
 #[diag(lint_unused_comparisons)]
 pub(crate) struct UnusedComparisons;