Rollup merge of #100460 - cuviper:drop-llvm-12, r=nagisa

Update the minimum external LLVM to 13

With this change, we'll have stable support for LLVM 13 through 15 (pending release).
For reference, the previous increase to LLVM 12 was #90175.

r? `@nagisa`

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2424,12 +2424,6 @@ extern "C" {
         cookie_out: &mut c_uint,
     ) -> &'a SMDiagnostic;
 
-    pub fn LLVMRustSetInlineAsmDiagnosticHandler(
-        C: &Context,
-        H: InlineAsmDiagHandlerTy,
-        CX: *mut c_void,
-    );
-
     #[allow(improper_ctypes)]
     pub fn LLVMRustUnpackSMDiagnostic(
         d: &SMDiagnostic,