ms-compat: Fix taking the address of a member of a dependent base

If unqualified id lookup fails while parsing a class template with a
dependent base, clang with -fms-compatibility will pretend the user
prefixed the name with 'this->' in order to delay the lookup.  However,
if there was a unary ampersand, Sema::ActOnDependentIdExpression() will
create a DependentDeclRefExpr, which is not what we wanted at all.  Fix
this by building the CXXDependentScopeMemberExpr directly instead.

In order to be fully MSVC compatible, we would have to defer all
attempts at name lookup to instantiation time.  However, until we have
real problems with system headers that can't be parsed, we'll put off
implementing that.

Fixes PR16014.

Reviewers: rsmith

Differential Revision: http://llvm-reviews.chandlerc.com/D1892

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192727 91177308-0d34-0410-b5e6-96231b3b80d8
Add llvm_unreachable to pacify GCC's dropping off non-void function warning.

+ 80 cols fix.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192728 91177308-0d34-0410-b5e6-96231b3b80d8