Fix crash in pinning autocorrect (sorbet#3924)

* Add ENFORCE that would have made error more obvious

* Add failing test

Fails with this backtrace:

    Exception::raise(): core/Loc.cc:270 enforced condition exists() has failed: Can't take source of Loc that doesn't exist

    Backtrace:
      #3 0x1725725 sorbet::core::Loc::source()
      #4 0x120e0e1 sorbet::infer::Environment::processBinding()
      #5 0x11fae5b sorbet::infer::Inference::run()
      #6 0xed1a88 sorbet::realmain::pipeline::CFGCollectorAndTyper::preTransformMethodDef()
      #7 0xed185b sorbet::ast::CALL_MEMBER_impl_preTransformMethodDef<>::call<>()
      #8 0xecf90c sorbet::ast::TreeMapper<>::mapMethodDef()
      #9 0xece3ff sorbet::ast::TreeMapper<>::mapIt()
      #10 0xecf81f sorbet::ast::TreeMapper<>::mapClassDef()
      #11 0xece37a sorbet::ast::TreeMapper<>::mapIt()
      #12 0xed1409 sorbet::ast::TreeMapper<>::mapInsSeq()
      #13 0xecef9f sorbet::ast::TreeMapper<>::mapIt()
      #14 0xebc5d9 sorbet::ast::TreeMap::apply<>()
      #15 0xea2971 sorbet::realmain::pipeline::typecheckOne()
      #16 0xeb3dc6 sorbet::realmain::pipeline::typecheck()::$_4::operator()()
      #17 0xeb3acd std::__1::__invoke<>()
      #18 0xeb3a7d std::__1::__invoke_void_return_wrapper<>::__call<>()
      #19 0xeb3a4d std::__1::__function::__alloc_func<>::operator()()
      #20 0xeb2bee std::__1::__function::__func<>::operator()()
      #21 0x177e015 std::__1::__function::__value_func<>::operator()()
      #22 0x177ddb5 std::__1::function<>::operator()()
      #23 0x17fc90e sorbet::WorkerPoolImpl::multiplexJob()
      #24 0xea5633 sorbet::realmain::pipeline::typecheck()
      #25 0xafb232 sorbet::realmain::realmain()
      #26 0xaf70f2 main
      #27 0x7fba7de990b3 __libc_start_main
      #28 0xaf702e _start

* Fix failing test

Author: jez

core/Loc.cc

@@ -273,6 +273,7 @@ string Loc::filePosToString(const GlobalState &gs, bool showFull) const {
 }
 
 string Loc::source(const GlobalState &gs) const {
+    ENFORCE(exists(), "Can't take source of Loc that doesn't exist");
     auto source = this->file().data(gs).source();
     return string(source.substr(beginPos(), endPos() - beginPos()));
 }

infer/environment.cc

@@ -1388,7 +1388,7 @@ core::TypePtr Environment::processBinding(core::Context ctx, const cfg::CFG &inW
                                 e.addErrorSection(core::ErrorSection(core::ErrorColors::format(
                                     "Attempting to change type to: `{}`\n", tp.type.show(ctx))));
 
-                                if (cur.origins.size() == 1) {
+                                if (cur.origins.size() == 1 && cur.origins[0].exists()) {
                                     // NOTE(nelhage): We assume that if there is
                                     // a single definition location, that
                                     // corresponds to an initial

test/testdata/infer/suggest_t_let_self.rb

@@ -0,0 +1,7 @@
+# typed: true
+
+x = self
+
+1.times do
+  x = 10 # error: Changing the type of a variable in a loop is not permitted
+end