Add an optional field attached to a DeclRefExpr which points back to the

Decl actually found via name lookup & overload resolution when that Decl
is different from the ValueDecl which is actually referenced by the
expression.

This can be used by AST consumers to correctly attribute references to
the spelling location of a using declaration, and otherwise gain insight
into the name resolution performed by Clang.

The public interface to DRE is kept as narrow as possible: we provide
a getFoundDecl() which always returns a NamedDecl, either the ValueDecl
referenced or the new, more precise NamedDecl if present. This way AST
clients can code against getFoundDecl without know when exactly the AST
has a split representation.

For an example of the data this provides consider:
% cat x.cc
namespace N1 {
  struct S {};
  void f(const S&);
}
void test(N1::S s) {
  f(s);
  using N1::f;
  f(s);
}

% ./bin/clang -fsyntax-only -Xclang -ast-dump x.cc
[...]
void test(N1::S s) (CompoundStmt 0x5b02010 <x.cc:5:20, line:9:1>
  (CallExpr 0x5b01df0 <line:6:3, col:6> 'void'
    (ImplicitCastExpr 0x5b01dd8 <col:3> 'void (*)(const struct N1::S &)' <FunctionToPointerDecay>
      (DeclRefExpr 0x5b01d80 <col:3> 'void (const struct N1::S &)' lvalue Function 0x5b01a20 'f' 'void (const struct N1::S &)'))
    (ImplicitCastExpr 0x5b01e20 <col:5> 'const struct N1::S' lvalue <NoOp>
      (DeclRefExpr 0x5b01d58 <col:5> 'N1::S':'struct N1::S' lvalue ParmVar 0x5b01b60 's' 'N1::S':'struct N1::S')))
  (DeclStmt 0x5b01ee0 <line:7:3, col:14>
    0x5b01e40 "UsingN1::;")
  (CallExpr 0x5b01fc8 <line:8:3, col:6> 'void'
    (ImplicitCastExpr 0x5b01fb0 <col:3> 'void (*)(const struct N1::S &)' <FunctionToPointerDecay>
      (DeclRefExpr 0x5b01f80 <col:3> 'void (const struct N1::S &)' lvalue Function 0x5b01a20 'f' 'void (const struct N1::S &)' (UsingShadow 0x5b01ea0 'f')))
    (ImplicitCastExpr 0x5b01ff8 <col:5> 'const struct N1::S' lvalue <NoOp>
      (DeclRefExpr 0x5b01f58 <col:5> 'N1::S':'struct N1::S' lvalue ParmVar 0x5b01b60 's' 'N1::S':'struct N1::S'))))

Now we can tell that the second call is 'using' (no pun intended) the using
declaration, and *which* using declaration it sees. Without this, we can
mistake calls that go through using declarations for ADL calls, and have no way
to attribute names looked up with using declarations to the appropriate
UsingDecl.

llvm-svn: 130670