[lldb][test] Add test for ASTImporter's name conflict resolution

This is a reduced test case from a crash we've observed in the past.
The assertion that this test triggers is:
```
Assertion failed: ((Pos == ImportedDecls.end() || Pos->second == To) && "Try to import an already imported Decl"), function MapImported, file ASTImporter.cpp, line 10494.
```

In a non-asserts build we crash later on in the ASTImporter. The
root cause is, as the assertion above points out, that we erroneously
replace an existing `From->To` decl mapping with a `To` decl that
isn't complete. Then we try to complete it but it has no definition
and we dereference a nullptr.

The reason this happens is basically what's been described in https://reviews.llvm.org/D67803?id=220956#1676588

The dylib contains a definition of `Service` which is different to the
one in the main executable. When we start dumping the children of the
variable we're printing, we start completing it's members, `ASTImport`ing
fields in the process. When the ASTImporter realizes there's been a name
conflict (i.e., a structural mismatch on the `Service` type) it would
usually report back an error. However, LLDB uses `ODRHandlingType::Liberal`,
which means we create a new decl for the ODR'd type instead of re-using
the previously mapped decl. Eventually this leads us to crash.

Ideally we'd be using `ODRHandlingType::Conservative` and warn/error, though
LLDB relies on this in some cases (particularly for distinguishing template
specializations, though maybe there's better a way to deal with those).

We should really warn the user when this happens and not crash. To avoid
the crash we'd need to know to not create a decl for the ODR violation,
and instead re-use the definition we've previously seen. Though I'm not
yet sure that's viable for all of LLDB's use-cases (where ODR violations
might legimiately occur in a program, e.g., with opaque definitions,
etc.).

Author: Michael137

lldb/test/Shell/Expr/Inputs/name-conflict-test/main.cpp

@@ -0,0 +1,21 @@
+#include "service.h"
+#include <cassert>
+#include <dlfcn.h>
+
+#ifndef PLUGIN_PATH
+#error "Expected PLUGIN_PATH to be defined"
+#endif // !PLUGIN_PATH
+
+int main() {
+  void *handle = dlopen(PLUGIN_PATH, RTLD_NOW);
+  assert(handle != nullptr);
+  void (*plugin_init)(void) = (void (*)(void))dlsym(handle, "plugin_init");
+  assert(plugin_init != nullptr);
+  void (*plugin_entry)(void) = (void (*)(void))dlsym(handle, "plugin_entry");
+  assert(plugin_entry != nullptr);
+
+  exported();
+  plugin_init();
+  plugin_entry();
+  return 0;
+}

lldb/test/Shell/Expr/Inputs/name-conflict-test/plugin.cpp

@@ -0,0 +1,15 @@
+#include "service.h"
+#include "plugin.h"
+
+struct Proxy : public Service {
+  State *proxyState;
+};
+
+Proxy *gProxyThis = 0;
+
+extern "C" {
+void plugin_init() { gProxyThis = new Proxy; }
+
+void plugin_entry() {}
+}
+

lldb/test/Shell/Expr/Inputs/name-conflict-test/plugin.h

@@ -0,0 +1,10 @@
+#ifndef PLUGIN_H_IN
+#define PLUGIN_H_IN
+
+extern "C" {
+void plugin_entry(void);
+void plugin_init(void);
+}
+
+#endif // _H_IN
+

lldb/test/Shell/Expr/Inputs/name-conflict-test/service.cpp

@@ -0,0 +1,15 @@
+#include "service.h"
+
+struct ServiceAux {
+  Service *Owner;
+};
+
+struct Service::State {};
+
+void exported() {
+  // Make sure debug-info for definition of Service is
+  // emitted in this CU.
+  Service service;
+  service.start(0);
+}
+

lldb/test/Shell/Expr/Inputs/name-conflict-test/service.h

@@ -0,0 +1,20 @@
+#ifndef SERVICE_H_IN
+#define SERVICE_H_IN
+
+struct ServiceAux;
+
+struct Service {
+  struct State;
+  bool start(State *) { return true; }
+
+#if HIDE_FROM_PLUGIN
+  int __resv1;
+#endif // !HIDE_FROM_PLUGIN
+
+  Service *__owner;
+  ServiceAux *aux;
+};
+
+void exported();
+
+#endif

lldb/test/Shell/Expr/TestODRHandlingWithDylib.test

@@ -0,0 +1,17 @@
+# REQUIRES: system-darwin
+
+# RUN: %clangxx_host %p/Inputs/name-conflict-test/plugin.cpp -shared -gdwarf -O0 -o %t.plugin.dylib
+#
+# RUN: %clangxx_host %p/Inputs/name-conflict-test/main.cpp \
+# RUN:               %p/Inputs/name-conflict-test/service.cpp \
+# RUN:               -DPLUGIN_PATH=\"%t.plugin.dylib\" \
+# RUN:               -DHIDE_FROM_PLUGIN \
+# RUN:               -gdwarf -O0 -o %t.a.out
+#
+# RUN: %lldb %t.a.out \
+# RUN:     -o "b plugin_entry" \
+# RUN:     -o run \
+# RUN:     -o "expr *gProxyThis" \
+# RUN:     -o exit | FileCheck  %s
+
+# CHECK: (lldb) expr *gProxyThis