implement PR4451, improving error recovery for a mistaken : where a :: was

intended.  On the first testcase in the bug, we now produce:

cxx-decl.cpp:12:2: error: unexpected ':' in nested name specifier
y:a a2;
 ^
 ::

instead of:

t.cc:8:1: error: C++ requires a type specifier for all declarations
x:a a2;
^
t.cc:8:2: error: invalid token after top level declarator
x:a a2;
 ^
 ;
t.cc:9:11: error: use of undeclared identifier 'a2'
x::a a3 = a2;
          ^

llvm-svn: 90713