[AST] Add generator for source location introspection

Generate a json file containing descriptions of AST classes and their
public accessors which return SourceLocation or SourceRange.

Use the JSON file to generate a C++ API and implementation for accessing
the source locations and method names for accessing them for a given AST
node.

This new API can be used to implement 'srcloc' output in clang-query:

  http://ce.steveire.com/z/m_kTIo

In this first version of this feature, only the accessors for Stmt
classes are generated, not Decls, TypeLocs etc.  Those can be added
after this change is reviewed, as this change is mostly about
infrastructure of these code generators.

Differential Revision: https://reviews.llvm.org/D93164