Clang: Improve locking of string cache

The string cache is only very seldom written but very often read. To
improve thread scaling it is faster to lock it only for write operations.
So we use a shared mutex which is locked in shared mode for read
operations and locked exclusively for write operations.

Change-Id: I2cc742d1b9cc15c162be40ab339fa8310640bc44
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>