lldb-server: rename `llgs::terminate` (NFC)

`terminate` overlaps with a C function on Windows:

```
D:\a\1\s\lldb\tools\lldb-server\lldb-server.cpp(45,13):  warning: 'terminate' redeclared without 'dllimport' attribute: 'dllexport' attribute added [-Winconsistent-dllimport]
static void terminate() { g_debugger_lifetime->Terminate(); }
            ^
C:\Program Files (x86)\Windows Kits\10\include\10.0.17763.0\ucrt\corecrt_terminate.h(29,48):  note: previous declaration is here
    _ACRTIMP __declspec(noreturn) void __cdecl terminate() throw();
                                               ^
D:\a\1\s\lldb\tools\lldb-server\lldb-server.cpp(45,61):  warning: function declared 'noreturn' should not return [-Winvalid-noreturn]
static void terminate() { g_debugger_lifetime->Terminate(); }
                                                            ^
```

Rename the function to `terminate_debugger` to avoid the errant match.

llvm-svn: 360693