When the parser is live, print out the location and spelling of its current token.

For example:

Stack dump:
0.	Program arguments: clang t.cpp 
1.	t.cpp:4:8: current parser token: ';'
2.	t.cpp:3:1: parsing struct/union/class body 'x'
Abort

It is weird that the parser is always "underneath" any parse context 
actions, but the parser is created first.

llvm-svn: 66148