[Sema] Fix decltype of static data members

Summary:
According to the C++11 standard [dcl.type.simple]p4:
  The type denoted by decltype(e) is defined as follows:
  - if e is an unparenthesized id-expression or an unparenthesized
    class member access (5.2.5), decltype(e) is the type of the entity
    named by e.
    
Currently Clang handles the 'member access' case incorrectly for
static data members (decltype returns T& instead of T). This patch
fixes the issue.

Reviewers: faisalv, rsmith, rogfer01

Reviewed By: rogfer01

Subscribers: rogfer01, cfe-commits

Differential Revision: https://reviews.llvm.org/D42969

llvm-svn: 325117