[lldb] Clear thread name container before writing UTF8 bytes (llvm#134150)

`llvm::convertUTF16ToUTF8String` opens with an assertion that the output
container is empty:
https://github.com/llvm/llvm-project/blob/3bdf9a08804a5b424fd32fef3b0089f3a6db839d/llvm/lib/Support/ConvertUTFWrapper.cpp#L83-L84

It's not clear to me why this function requires the output container to
be empty instead of just overwriting it, but the callsite in
`TargetThreadWindows::GetName` may reuse the container without clearing
it out first, resulting in an assertion failure:
```
 # Child-SP          RetAddr               Call Site
00 000000d2`44b8ea48 00007ff8`beefc12e     ntdll!NtTerminateProcess+0x14
01 000000d2`44b8ea50 00007ff8`bcf518ab     ntdll!RtlExitUserProcess+0x11e
02 000000d2`44b8ea80 00007ff8`bc0e0143     KERNEL32!ExitProcessImplementation+0xb
03 000000d2`44b8eab0 00007ff8`bc0e4c49     ucrtbase!common_exit+0xc7
04 000000d2`44b8eb10 00007ff8`bc102ae6     ucrtbase!abort+0x69
05 000000d2`44b8eb40 00007ff8`bc102cc1     ucrtbase!common_assert_to_stderr<wchar_t>+0x6e
06 000000d2`44b8eb80 00007fff`b8e27a80     ucrtbase!wassert+0x71
07 000000d2`44b8ebb0 00007fff`b8b821e1     liblldb!llvm::convertUTF16ToUTF8String+0x30 [D:\r\_work\swift-build\swift-build\SourceCache\llvm-project\llvm\lib\Support\ConvertUTFWrapper.cpp @ 88] 
08 000000d2`44b8ec30 00007fff`b83e9aa2     liblldb!lldb_private::TargetThreadWindows::GetName+0x1b1 [D:\r\_work\swift-build\swift-build\SourceCache\llvm-project\lldb\source\Plugins\Process\Windows\Common\TargetThreadWindows.cpp @ 198] 
09 000000d2`44b8eca0 00007ff7`2a3c3c14     liblldb!lldb::SBThread::GetName+0x102 [D:\r\_work\swift-build\swift-build\SourceCache\llvm-project\lldb\source\API\SBThread.cpp @ 432] 
0a 000000d2`44b8ed70 00007ff7`2a3a5ac6     lldb_dap!lldb_dap::CreateThread+0x1f4 [S:\SourceCache\llvm-project\lldb\tools\lldb-dap\JSONUtils.cpp @ 877] 
0b 000000d2`44b8ef10 00007ff7`2a3b0ab5     lldb_dap!`anonymous namespace'::request_threads+0xa6 [S:\SourceCache\llvm-project\lldb\tools\lldb-dap\lldb-dap.cpp @ 3906] 
0c 000000d2`44b8f010 00007ff7`2a3b0fe8     lldb_dap!lldb_dap::DAP::HandleObject+0x1c5 [S:\SourceCache\llvm-project\lldb\tools\lldb-dap\DAP.cpp @ 796] 
0d 000000d2`44b8f130 00007ff7`2a3a8b96     lldb_dap!lldb_dap::DAP::Loop+0x78 [S:\SourceCache\llvm-project\lldb\tools\lldb-dap\DAP.cpp @ 812] 
0e 000000d2`44b8f1d0 00007ff7`2a4b5fbc     lldb_dap!main+0x1096 [S:\SourceCache\llvm-project\lldb\tools\lldb-dap\lldb-dap.cpp @ 5319] 
0f (Inline Function) --------`--------     lldb_dap!invoke_main+0x22 [D:\a\_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl @ 78] 
10 000000d2`44b8fb80 00007ff8`bcf3e8d7     lldb_dap!__scrt_common_main_seh+0x10c [D:\a\_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl @ 288] 
11 000000d2`44b8fbc0 00007ff8`beefbf6c     KERNEL32!BaseThreadInitThunk+0x17
12 000000d2`44b8fbf0 00000000`00000000     ntdll!RtlUserThreadStart+0x2c
```

This stack trace was captured from the lldb distributed in the Swift
toolchain. The issue is easy to reproduce by resuming from a breakpoint
twice in VS Code.

I've verified that clearing out the container here fixes the assertion
failure.

Author: z2oh

lldb/source/Plugins/Process/Windows/Common/TargetThreadWindows.cpp

@@ -192,6 +192,7 @@ const char *TargetThreadWindows::GetName() {
   if (SUCCEEDED(GetThreadDescription(
           m_host_thread.GetNativeThread().GetSystemHandle(), &pszThreadName))) {
     LLDB_LOGF(log, "GetThreadDescription: %ls", pszThreadName);
+    m_name.clear();
     llvm::convertUTF16ToUTF8String(
         llvm::ArrayRef(reinterpret_cast<char *>(pszThreadName),
                        wcslen(pszThreadName) * sizeof(wchar_t)),