Improved generator tracking

* Successive executions of the same `GENERATE` macro (e.g. because
of a for loop) no longer lead to multiple nested generators.
* The same line can now contain multiple `GENERATE` macros without
issues.

Fixes #1913

examples/311-Gen-CustomCapture.cpp

@@ -29,7 +29,7 @@ TEST_CASE("Generate random doubles across different ranges",
     // This will take r1 by reference and r2 by value.
     // Note that there are no advantages for doing so in this example,
     // it is done only for expository purposes.
-    auto number = Catch::Generators::generate( CATCH_INTERNAL_LINEINFO,
+    auto number = Catch::Generators::generate( "custom capture generator", CATCH_INTERNAL_LINEINFO,
         [&r1, r2]{
             using namespace Catch::Generators;
             return makeGenerators(take(50, random(std::get<0>(r1), std::get<1>(r2))));