Account for macros when trying to point at inference cause

Do not point at macro invocation which expands to an inference error. Avoid the following:

```
error[E0308]: mismatched types
  --> $DIR/does-not-have-iter-interpolated.rs:12:5
   |
LL |     quote!($($nonrep)*);
   |     ^^^^^^^^^^^^^^^^^^^
   |     |
   |     expected `HasIterator`, found `ThereIsNoIteratorInRepetition`
   |     expected due to this
   |     here the type of `has_iter` is inferred to be `ThereIsNoIteratorInRepetition`
```

Author: estebank

compiler/rustc_hir_typeck/src/demand.rs

@@ -698,7 +698,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     ) {
         match (self.tcx.parent_hir_node(expr.hir_id), error) {
             (hir::Node::LetStmt(hir::LetStmt { ty: Some(ty), init: Some(init), .. }), _)
-                if init.hir_id == expr.hir_id =>
+                if init.hir_id == expr.hir_id && !ty.span.source_equal(init.span) =>
             {
                 // Point at `let` assignment type.
                 err.span_label(ty.span, "expected due to this");

tests/ui/proc-macro/quote/does-not-have-iter-interpolated-dup.stderr

@@ -5,7 +5,6 @@ LL |     quote!($($nonrep $nonrep)*);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |     |
    |     expected `HasIterator`, found `ThereIsNoIteratorInRepetition`
-   |     expected due to this
    |     here the type of `has_iter` is inferred to be `ThereIsNoIteratorInRepetition`
 
 error: aborting due to 1 previous error

tests/ui/proc-macro/quote/does-not-have-iter-interpolated.stderr

@@ -5,7 +5,6 @@ LL |     quote!($($nonrep)*);
    |     ^^^^^^^^^^^^^^^^^^^
    |     |
    |     expected `HasIterator`, found `ThereIsNoIteratorInRepetition`
-   |     expected due to this
    |     here the type of `has_iter` is inferred to be `ThereIsNoIteratorInRepetition`
 
 error: aborting due to 1 previous error

tests/ui/proc-macro/quote/does-not-have-iter-separated.stderr

@@ -2,10 +2,7 @@ error[E0308]: mismatched types
   --> $DIR/does-not-have-iter-separated.rs:8:5
    |
 LL |     quote!($(a b),*);
-   |     ^^^^^^^^^^^^^^^^
-   |     |
-   |     expected `HasIterator`, found `ThereIsNoIteratorInRepetition`
-   |     expected due to this
+   |     ^^^^^^^^^^^^^^^^ expected `HasIterator`, found `ThereIsNoIteratorInRepetition`
 
 error: aborting due to 1 previous error
 

tests/ui/proc-macro/quote/does-not-have-iter.stderr

@@ -2,10 +2,7 @@ error[E0308]: mismatched types
   --> $DIR/does-not-have-iter.rs:8:5
    |
 LL |     quote!($(a b)*);
-   |     ^^^^^^^^^^^^^^^
-   |     |
-   |     expected `HasIterator`, found `ThereIsNoIteratorInRepetition`
-   |     expected due to this
+   |     ^^^^^^^^^^^^^^^ expected `HasIterator`, found `ThereIsNoIteratorInRepetition`
 
 error: aborting due to 1 previous error