[REG 5.7.0 -> 5.7.1] Not detecting C++11 features of clang-cl

Change-Id: I149e0540c00745fe8119fffd146286ffe480d216 seems to have introduced this regression:

Building with Microsoft's compiler in MSVC2015 Update 3, the following macros are correctly defined by qcompilerdetection.h:

Q_COMPILER_DECLTYPE
Q_COMPILER_INITIALIZER_LISTS
Q_COMPILER_NULLPTR
Q_COMPILER_RVALUE_REFS
Q_COMPILER_CLASS_ENUM
Q_COMPILER_TEMPLATE_ALIAS
Q_COMPILER_VARIADIC_TEMPLATES

Building with clang-cl using Microsoft's implementation of the standard library in the very same version of MSVC, qcompilerdetection.h defines none of the above macros.

This makes it impossible to do something as simple as use braced initializer syntax with QList.

Qt 5.7.0 is unaffected.