Fix a reference-counting bug in ScopedSharedMutexReader.

When the copy constructor is called m_readers needs to be incremented
because the destructor will be calkled on both the original object and
the copy.

Author: adrian-prantl

lldb/include/lldb/Core/SwiftScratchContextReader.h

@@ -40,6 +40,7 @@ class SharedMutex {
   }
   bool unlock_shared() {
     std::lock_guard<std::mutex> lock(m_reader_mutex);
+    assert(m_readers > 0);
     --m_readers;
     return m_impl.unlock_shared();
   }
@@ -57,7 +58,12 @@ struct ScopedSharedMutexReader {
     if (m_mutex)
       m_mutex->lock_shared();
   }
-  ScopedSharedMutexReader(const ScopedSharedMutexReader&) = default;
+
+  ScopedSharedMutexReader(const ScopedSharedMutexReader &copy)
+      : m_mutex(copy.m_mutex) {
+    if (m_mutex)
+      m_mutex->lock_shared();
+  }
 
   ~ScopedSharedMutexReader() {
     if (m_mutex)
@@ -101,9 +107,6 @@ class SwiftScratchContextReader : ScopedSharedMutexReader {
     assert(m_ptr && "invalid context");
   }
 
-  SwiftScratchContextReader(const SwiftScratchContextReader &copy)
-      : ScopedSharedMutexReader(copy.m_mutex), m_ptr(copy.m_ptr) {}
-
   TypeSystemSwiftTypeRefForExpressions *get() {
     assert(m_ptr && "invalid context");
     return m_ptr;