Changes to the name lookup have caused a regression in the digraph fix-it hint.

Changes to the name lookup have caused a regression in the digraph fix-it hint.
For instance:

template <class T> void E() {};
class F {};

void test() {
 ::E<::F>();
 E<::F>();
}

Gives the following error messages:

error: found '<::' after a template name which forms the
     digraph '<:' (aka '[') and a ':', did you mean '< ::'?
 ::E<::F>();
    ^~~
    < ::
error: expected expression
 E<::F>();
    ^
error: expected ']'
note: to match this '['
 E<::F>();

This patch adds the digraph fix-it check right before the name lookup,
moves the shared checking code to a new function, and adds new
tests to catch future regressions.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@140039 91177308-0d34-0410-b5e6-96231b3b80d8