Add QualifiedDeclRefExpr, which retains additional source-location

Add QualifiedDeclRefExpr, which retains additional source-location
information for declarations that were referenced via a qualified-id,
e.g., N::C::value. We keep track of the location of the start of the
nested-name-specifier. Note that the difference between
QualifiedDeclRefExpr and DeclRefExpr does have an effect on the
semantics of function calls in two ways:
  1) The use of a qualified-id instead of an unqualified-id suppresses
     argument-dependent lookup
  2) If the name refers to a virtual function, the qualified-id
  version will call the function determined statically while the
  unqualified-id version will call the function determined dynamically
  (by looking up the appropriate function in the vtable).

Neither of these features is implemented yet, but we do print out
qualified names for QualifiedDeclRefExprs as part of the AST printing.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@61789 91177308-0d34-0410-b5e6-96231b3b80d8