Merge target and launch info environments

[JDevlieghere]

Summary

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.

[labath]

The fix looks good, but it seems like it wouldn't be hard to make the test windows-compatible, so I think we should do that. All it would take is to compile a very dumb version of "env" which just prints the value of the single env var. If you can get it to print in a different format then you can also avoid the double quote trick (which also doesn't work on windows).

Also, doesn't debugging of /usr/bin/env fail on darwin with SIP ?

lit/Process/Inputs/env.cpp

+#include <cstdlib>
+#include <iostream>
+
+int main() {
+  if (const char *env_p = std::getenv("FOO"))
+    std::cout << "FOO=" << env_p << '\n';
+}

lit/Process/TestEnvironment.test

+The double quotes around "BAR" ensure we don't match the command.
+
+RUN: %clangxx -std=c++11 %p/Inputs/env.cpp -o %t
+RUN: %lldb %t -o 'process launch --environment FOO="BAR"' | FileCheck %s
+RUN: %lldb %t -o 'env FOO="BAR"' -o 'process launch' | FileCheck %s
+
+CHECK: FOO=BAR

source/Commands/CommandObjectProcess.cpp

else
       m_options.launch_info.GetFlags().Clear(eLaunchFlagDisableASLR);
 
     if (target->GetDetachOnError())
       m_options.launch_info.GetFlags().Set(eLaunchFlagDetachOnError);
 
     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(
           target_settings_argv0);
       m_options.launch_info.SetExecutableFile(
           exe_module_sp->GetPlatformFileSpec(), false);
     } else {
       m_options.launch_info.SetExecutableFile(