Include namespace contexts in the virtual stack trace, so we get stuff

Include namespace contexts in the virtual stack trace, so we get stuff
like this:

Stack dump:
0.	using-directive.cpp:9:14: in compound statement ('{}')
1.	using-directive.cpp:9:14: parsing function body 'A::B::f'
2.	using-directive.cpp:7:3: parsing namespace 'A::B'
3.	using-directive.cpp:5:1: parsing namespace 'A'
4.	clang using-directive.cpp
Abort

for testcase like like:

namespace A {
  short i;
  namespace B {
    long i;
    void f() { <<crash>>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@66123 91177308-0d34-0410-b5e6-96231b3b80d8