Add check for kind of UnqualifiedId in Declarator::isStaticMember()

Method CXXMethodDecl::isStaticOverloadedOperator expects Operator field
from the struct OperatorFunctionId, which is a member of the union in
the class UnqualifiedId. If the kind of UnqualifiedId is not checked,
there is no guarantee that the value that this method receives will be
correct, because it can be the value of another union member and not
OperatorFunctionId.

This bug manifests itself when running make check-all on mips64 BE. 

This fix resolves the following regression tests: 
Clang :: CXX/special/class.dtor/p9.cpp
Clang :: CodeGenCXX/2006-09-12-OpaqueStructCrash.cpp
Clang :: CodeGenCXX/ctor-dtor-alias.cpp
Clang :: CodeGenCXX/debug-info-windows-dtor.cpp
Clang :: CodeGenCXX/dllexport-members.cpp
Clang :: CodeGenCXX/dllexport.cpp

Patch by Violeta Vukobrat.

Differential Revision:  http://reviews.llvm.org/D8437

llvm-svn: 233508