Introduce clang-query tool.

This tool is for interactive exploration of the Clang AST using AST matchers.
It currently allows the user to enter a matcher at an interactive prompt
and view the resulting bindings as diagnostics, AST pretty prints or AST
dumps. Example session:

$ cat foo.c
void foo(void) {}
$ clang-query foo.c --
clang-query> match functionDecl()

Match #1:

foo.c:1:1: note: "root" binds here
void foo(void) {}
^~~~~~~~~~~~~~~~~
1 match.

Differential Revision: http://llvm-reviews.chandlerc.com/D2098