Modify generator tracking to allow GENERATEs between SECTIONs

This means that code such as

```cpp
TEST_CASE() {
    SECTION("first") { SUCCEED(); }
    auto _ = GENERATE(1, 2);
    SECTION("second") { SUCCEED(); }
}
```

will run and report 3 assertions, 1 from section "first" and 2
from section "second". This also applies for greater and potentially
more confusing nesting, but fundamentally it is up to the user to
avoid overly complex and confusing nestings, just as with `SECTION`s.

The old behaviour of `GENERATE` as first thing in a `TEST_CASE`,
`GENERATE` not followed by a `SECTION`, etc etc should be unchanged.

Closes #1938

tests/SelfTest/IntrospectiveTests/PartTracker.tests.cpp

@@ -4,6 +4,7 @@
  */
 
 #include <catch2/catch_test_macros.hpp>
+#include <catch2/generators/catch_generators.hpp>
 #include <catch2/internal/catch_test_case_tracker.hpp>
 
 
@@ -201,3 +202,50 @@ TEST_CASE("#1670 regression check", "[.approvals][tracker]") {
         SECTION("2") SUCCEED();
     }
 }
+
+// #1938 required a rework on how generator tracking works, so that `GENERATE`
+// supports being sandwiched between two `SECTION`s. The following tests check
+// various other scenarios through checking output in approval tests.
+TEST_CASE("#1938 - GENERATE after a section", "[.][regression][generators]") {
+    SECTION("A") {
+        SUCCEED("A");
+    }
+    auto m = GENERATE(1, 2, 3);
+    SECTION("B") {
+        REQUIRE(m);
+    }
+}
+
+TEST_CASE("#1938 - flat generate", "[.][regression][generators]") {
+    auto m = GENERATE(1, 2, 3);
+    REQUIRE(m);
+}
+
+TEST_CASE("#1938 - nested generate", "[.][regression][generators]") {
+    auto m = GENERATE(1, 2, 3);
+    auto n = GENERATE(1, 2, 3);
+    REQUIRE(m);
+    REQUIRE(n);
+}
+
+TEST_CASE("#1938 - mixed sections and generates", "[.][regression][generators]") {
+    auto i = GENERATE(1, 2);
+    SECTION("A") {
+        SUCCEED("A");
+    }
+    auto j = GENERATE(3, 4);
+    SECTION("B") {
+        SUCCEED("B");
+    }
+    auto k = GENERATE(5, 6);
+    CAPTURE(i, j, k);
+    SUCCEED();
+}
+
+TEST_CASE("#1938 - Section followed by flat generate", "[.][regression][generators]") {
+    SECTION("A") {
+        REQUIRE(1);
+    }
+    auto m = GENERATE(2, 3);
+    REQUIRE(m);
+}