Merge target and launch info environments

Before this change we were overriding the launch info environment with
the target environment. This meant that the environment variables passed
to `process launch --environment <>` were lost. Instead of replacing the
environment, we should merge them.

Differential revision: https://reviews.llvm.org/D61864

llvm-svn: 360612
diff --git a/lldb/source/Commands/CommandObjectProcess.cpp b/lldb/source/Commands/CommandObjectProcess.cpp
index 1cc0b0b..b20a2d5 100644
--- a/lldb/source/Commands/CommandObjectProcess.cpp
+++ b/lldb/source/Commands/CommandObjectProcess.cpp
@@ -193,7 +193,10 @@
     if (target->GetDisableSTDIO())
       m_options.launch_info.GetFlags().Set(eLaunchFlagDisableSTDIO);
 
-    m_options.launch_info.GetEnvironment() = target->GetEnvironment();
+    // Merge the launch info environment with the target environment.
+    Environment target_env = target->GetEnvironment();
+    m_options.launch_info.GetEnvironment().insert(target_env.begin(),
+                                                  target_env.end());
 
     if (!target_settings_argv0.empty()) {
       m_options.launch_info.GetArguments().AppendArgument(