[lldb][Linux] Fix checking of error values when attach fails

Relates to #161510

(it's not fixing it, it's just making the error not be an
unhandled error)

When we fail to attach to a process we see if we can add more
information about why it happpened:
```
  if (status.GetError() == EPERM) {
    // Depending on the value of ptrace_scope, we can return a different
    // error that suggests how to fix it.
    return AddPtraceScopeNote(status.ToError());
  }
```
ToError creates a new error value and leaves the one in `status`
unchecked. `status`'s error is ok because it will be checked
by Status' destructor.

The problem happens in `AddPtraceScopeNote`. If we take certain
return paths, this new error, or the one we get when trying to
find the ptrace scope, may be unchecked on destruction when the
function returns.

To fix this, in AddPtraceScopeNote, consume any errors that
we are not going to return. Anything returned will be checked
by some caller.

Reproducing this failure mode is difficult but it can be faked
by calling AddPtraceScopeNote earlier. Which is what I did to
prove the concept of the problem.

Author: DavidSpickett

lldb/source/Plugins/Process/Linux/NativeProcessLinux.cpp

@@ -219,7 +219,8 @@ static llvm::Error AddPtraceScopeNote(llvm::Error original_error) {
   Expected<int> ptrace_scope = GetPtraceScope();
   if (auto E = ptrace_scope.takeError()) {
     Log *log = GetLog(POSIXLog::Process);
-    LLDB_LOG(log, "error reading value of ptrace_scope: {0}", E);
+    LLDB_LOG(log, "error reading value of ptrace_scope: {0}",
+             llvm::toString(std::move(E)));
 
     // The original error is probably more interesting than not being able to
     // read or interpret ptrace_scope.
@@ -230,6 +231,7 @@ static llvm::Error AddPtraceScopeNote(llvm::Error original_error) {
   switch (*ptrace_scope) {
   case 1:
   case 2:
+    llvm::consumeError(std::move(original_error));
     return llvm::createStringError(
         std::error_code(errno, std::generic_category()),
         "The current value of ptrace_scope is %d, which can cause ptrace to "
@@ -239,6 +241,7 @@ static llvm::Error AddPtraceScopeNote(llvm::Error original_error) {
         "https://www.kernel.org/doc/Documentation/security/Yama.txt.",
         *ptrace_scope);
   case 3:
+    llvm::consumeError(std::move(original_error));
     return llvm::createStringError(
         std::error_code(errno, std::generic_category()),
         "The current value of ptrace_scope is 3, which will cause ptrace to "