Don't lose empty where clause when pretty-printing

Previously, we would parse `struct Foo where;` and `struct Foo;`
identically, leading to an 'empty' `where` clause being omitted during
pretty printing. This will cause us to lose spans when proc-macros
involved, since we will have a collected `where` token that does not
appear in the pretty-printed item.

We now explicitly track the presence of a `where` token during parsing,
so that we can distinguish between `struct Foo where;` and `struct Foo;`
during pretty-printing

src/test/ui/proc-macro/empty-where-clause.rs

@@ -0,0 +1,18 @@
+// aux-build:test-macros.rs
+
+extern crate test_macros;
+use test_macros::recollect_attr;
+
+#[recollect_attr]
+struct FieldStruct where {
+    field: MissingType1 //~ ERROR cannot find
+}
+
+#[recollect_attr]
+struct TupleStruct(MissingType2) where; //~ ERROR cannot find
+
+enum MyEnum where {
+    Variant(MissingType3) //~ ERROR cannot find
+}
+
+fn main() {}

src/test/ui/proc-macro/empty-where-clause.stderr

@@ -0,0 +1,21 @@
+error[E0412]: cannot find type `MissingType1` in this scope
+  --> $DIR/empty-where-clause.rs:8:12
+   |
+LL |     field: MissingType1
+   |            ^^^^^^^^^^^^ not found in this scope
+
+error[E0412]: cannot find type `MissingType2` in this scope
+  --> $DIR/empty-where-clause.rs:12:20
+   |
+LL | struct TupleStruct(MissingType2) where;
+   |                    ^^^^^^^^^^^^ not found in this scope
+
+error[E0412]: cannot find type `MissingType3` in this scope
+  --> $DIR/empty-where-clause.rs:15:13
+   |
+LL |     Variant(MissingType3)
+   |             ^^^^^^^^^^^^ not found in this scope
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0412`.