[lldb][Windows] Fix compilation warnings

As seen on Linaro's Windows on Arm bot.

C:\Users\tcwg\llvm-worker\lldb-aarch64-windows\llvm-project\lldb\source\Host\windows\MainLoopWindows.cpp(80,25): warning: missing field 'InternalHigh' initializer [-Wmissing-field-initializers]
   80 |       OVERLAPPED ov = {0};
      |                         ^
C:\Users\tcwg\llvm-worker\lldb-aarch64-windows\llvm-project\lldb\source\Host\windows\MainLoopWindows.cpp(135,8): warning: 'WillPoll' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
  135 |   void WillPoll() {
      |        ^
C:\Users\tcwg\llvm-worker\lldb-aarch64-windows\llvm-project\lldb\include\lldb/Host/windows/MainLoopWindows.h(40,18): note: overridden virtual function is here
   40 |     virtual void WillPoll() {}
      |                  ^
C:\Users\tcwg\llvm-worker\lldb-aarch64-windows\llvm-project\lldb\source\Host\windows\MainLoopWindows.cpp(142,8): warning: 'DidPoll' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
  142 |   void DidPoll() {
      |        ^
C:\Users\tcwg\llvm-worker\lldb-aarch64-windows\llvm-project\lldb\include\lldb/Host/windows/MainLoopWindows.h(41,18): note: overridden virtual function is here
   41 |     virtual void DidPoll() {}
      |                  ^

C:\Users\tcwg\llvm-worker\lldb-aarch64-windows\llvm-project\lldb\source\Host\windows\MainLoopWindows.cpp(80,25): warning: missing field 'InternalHigh' initializer [-Wmissing-field-initializers]
   80 |       OVERLAPPED ov = {0};
      |                         ^

Author: DavidSpickett

lldb/source/Host/windows/MainLoopWindows.cpp

@@ -77,7 +77,8 @@ class PipeEvent : public MainLoopWindows::IOEvent {
     do {
       char buf[1];
       DWORD bytes_read = 0;
-      OVERLAPPED ov = {0};
+      OVERLAPPED ov;
+      ZeroMemory(&ov, sizeof(ov));
       // Block on a 0-byte read; this will only resume when data is
       // available in the pipe. The pipe must be PIPE_WAIT or this thread
       // will spin.
@@ -132,14 +133,14 @@ class SocketEvent : public MainLoopWindows::IOEvent {
 
   ~SocketEvent() override { WSACloseEvent(m_event); }
 
-  void WillPoll() {
+  void WillPoll() override {
     int result =
         WSAEventSelect(m_socket, m_event, FD_READ | FD_ACCEPT | FD_CLOSE);
     assert(result == 0);
     UNUSED_IF_ASSERT_DISABLED(result);
   }
 
-  void DidPoll() {
+  void DidPoll() override {
     int result = WSAEventSelect(m_socket, WSA_INVALID_EVENT, 0);
     assert(result == 0);
     UNUSED_IF_ASSERT_DISABLED(result);