refactor: Do not discard try_lock() return value

Microsoft's C++ Standard Library uses the `[[nodiscard]]` attribute for
`try_lock()`.
See: https://github.com/microsoft/STL/blob/main/stl/inc/mutex

This change allows to drop the current suppression for the warning C4838
and helps to prevent the upcoming warning C4858.
See: microsoft/STL@539c26c

Author: hebasto

src/sync.h

@@ -165,8 +165,7 @@ class SCOPED_LOCKABLE UniqueLock : public Base
     bool TryEnter(const char* pszName, const char* pszFile, int nLine)
     {
         EnterCritical(pszName, pszFile, nLine, Base::mutex(), true);
-        Base::try_lock();
-        if (!Base::owns_lock()) {
+        if (!Base::try_lock()) {
             LeaveCritical();
         }
         return Base::owns_lock();