[lldb][TypeSystem] ForEach: Don't hold the TypeSystemMap lock across callback

The `TypeSystemMap::m_mutex` guards against concurrent modifications
of members of `TypeSystemMap`. In particular, `m_map`.

`TypeSystemMap::ForEach` iterates through the entire `m_map` calling
a user-specified callback for each entry. This is all done while
`m_mutex` is locked. However, there's nothing that guarantees that
the callback itself won't call back into `TypeSystemMap` APIs on the
same thread. This lead to double-locking `m_mutex`, which is undefined
behaviour. We've seen this cause a deadlock in the swift plugin with
following backtrace:

```

int main() {
    std::unique_ptr<int> up = std::make_unique<int>(5);

    volatile int val = *up;
    return val;
}

clang++ -std=c++2a -g -O1 main.cpp

./bin/lldb -o “br se -p return” -o run -o “v *up” -o “expr *up” -b
```

```
frame #4: std::lock_guard<std::mutex>::lock_guard
frame intel#5: lldb_private::TypeSystemMap::GetTypeSystemForLanguage <<<< Lock #2
frame intel#6: lldb_private::TypeSystemMap::GetTypeSystemForLanguage
frame intel#7: lldb_private::Target::GetScratchTypeSystemForLanguage
...
frame intel#26: lldb_private::SwiftASTContext::LoadLibraryUsingPaths
frame intel#27: lldb_private::SwiftASTContext::LoadModule
frame intel#30: swift::ModuleDecl::collectLinkLibraries
frame intel#31: lldb_private::SwiftASTContext::LoadModule
frame intel#34: lldb_private::SwiftASTContext::GetCompileUnitImportsImpl
frame intel#35: lldb_private::SwiftASTContext::PerformCompileUnitImports
frame intel#36: lldb_private::TypeSystemSwiftTypeRefForExpressions::GetSwiftASTContext
frame intel#37: lldb_private::TypeSystemSwiftTypeRefForExpressions::GetPersistentExpressionState
frame intel#38: lldb_private::Target::GetPersistentSymbol
frame intel#41: lldb_private::TypeSystemMap::ForEach                 <<<< Lock #1
frame intel#42: lldb_private::Target::GetPersistentSymbol
frame intel#43: lldb_private::IRExecutionUnit::FindInUserDefinedSymbols
frame intel#44: lldb_private::IRExecutionUnit::FindSymbol
frame intel#45: lldb_private::IRExecutionUnit::MemoryManager::GetSymbolAddressAndPresence
frame intel#46: lldb_private::IRExecutionUnit::MemoryManager::findSymbol
frame intel#47: non-virtual thunk to lldb_private::IRExecutionUnit::MemoryManager::findSymbol
frame intel#48: llvm::LinkingSymbolResolver::findSymbol
frame intel#49: llvm::LegacyJITSymbolResolver::lookup
frame intel#50: llvm::RuntimeDyldImpl::resolveExternalSymbols
frame intel#51: llvm::RuntimeDyldImpl::resolveRelocations
frame intel#52: llvm::MCJIT::finalizeLoadedModules
frame intel#53: llvm::MCJIT::finalizeObject
frame intel#54: lldb_private::IRExecutionUnit::ReportAllocations
frame intel#55: lldb_private::IRExecutionUnit::GetRunnableInfo
frame intel#56: lldb_private::ClangExpressionParser::PrepareForExecution
frame intel#57: lldb_private::ClangUserExpression::TryParse
frame intel#58: lldb_private::ClangUserExpression::Parse
```

Our solution is to simply iterate over a local copy of `m_map`.

**Testing**

* Confirmed on manual reproducer (would reproduce 100% of the time
  before the patch)

Differential Revision: https://reviews.llvm.org/D149949

Author: Michael137

lldb/source/Symbol/TypeSystem.cpp

@@ -217,11 +217,21 @@ void TypeSystemMap::Clear() {
 
 void TypeSystemMap::ForEach(
     std::function<bool(lldb::TypeSystemSP)> const &callback) {
-  std::lock_guard<std::mutex> guard(m_mutex);
+
+  // The callback may call into this function again causing
+  // us to lock m_mutex twice if we held it across the callback.
+  // Since we just care about guarding access to 'm_map', make
+  // a local copy and iterate over that instead.
+  collection map_snapshot;
+  {
+      std::lock_guard<std::mutex> guard(m_mutex);
+      map_snapshot = m_map;
+  }
+
   // Use a std::set so we only call the callback once for each unique
   // TypeSystem instance.
   llvm::DenseSet<TypeSystem *> visited;
-  for (auto &pair : m_map) {
+  for (auto &pair : map_snapshot) {
     TypeSystem *type_system = pair.second.get();
     if (!type_system || visited.count(type_system))
       continue;