Debugger: improve dump time for various msvc std containers

... by caching the info whether we have a debug build. This avoids
expanding members of the container to check for a potential offset of
the actual data.

Change-Id: I4d7ac83003c303185ae51d57f0f0571b627edf22
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

share/qtcreator/debugger/dumper.py

@@ -167,6 +167,7 @@ class DumperBase():
         self.isGdb = False
         self.isLldb = False
         self.isCli = False
+        self.isDebugBuild = None
 
         # Later set, or not set:
         self.stringCutOff = 10000