[lldb] common completion for process pids and process names

1. Added two common completions: `ProcessIDs` and `ProcessNames`, which are
refactored from their original dedicated option completions;
2. Removed the dedicated option completion functions of `process attach` and
`platform process attach`, so that they can use arg-type-bound common
completions instead;
3. Bound `eArgTypePid` to the pid completion, `eArgTypeProcessName` to the
process name completion in `CommandObject.cpp`;
4. Added a related test case.

Reviewed By: teemperor

Differential Revision: https://reviews.llvm.org/D80700

lldb/test/API/functionalities/completion/main.cpp

@@ -1,3 +1,5 @@
+#include <iostream>
+
 class Foo
 {
 public:
@@ -11,14 +13,16 @@ namespace { int Quux (void) { return 0; } }
 
 struct Container { int MemberVar; };
 
-int main()
-{
-    Foo fooo;
-    Foo *ptr_fooo = &fooo;
-    fooo.Bar(1, 2);
+int main(int argc, char *argv[]) {
+  if (argc > 1 && std::string(argv[1]) == "-x")
+    std::cin.get();
 
-    Container container;
-    Container *ptr_container = &container;
-    int q = Quux();
-    return container.MemberVar = 3; // Break here
+  Foo fooo;
+  Foo *ptr_fooo = &fooo;
+  fooo.Bar(1, 2);
+
+  Container container;
+  Container *ptr_container = &container;
+  int q = Quux();
+  return container.MemberVar = 3; // Break here
 }