[clang-repl] Disambiguate global namespace identifiers

A double colon starts an identifier name in the global namespace and
must be tentatively parsed as such.

Differential Revision: https://reviews.llvm.org/D157480

clang/lib/Parse/ParseTentative.cpp

@@ -62,6 +62,7 @@ bool Parser::isCXXDeclarationStatement(
   case tok::kw_static_assert:
   case tok::kw__Static_assert:
     return true;
+  case tok::coloncolon:
   case tok::identifier: {
     if (DisambiguatingWithExpression) {
       RevertingTentativeParsingAction TPA(*this);

clang/test/Interpreter/disambiguate-decl-stmt.cpp

@@ -38,6 +38,10 @@ struct Dtor1 {~Dtor1();};
 Dtor1::~Dtor1() { printf("Dtor1\n"); }
 Dtor1 d1;
 
+struct Dtor2 { ~Dtor2(); };
+::Dtor2::~Dtor2() { printf("Dtor2\n"); }
+Dtor2 d2;
+
 struct ANestedDtor { struct A1 { struct A2 { ~A2(); }; }; };
 ANestedDtor::A1::A2::~A2() { printf("Dtor A::A1::A2::~A2\n"); }