Add an AST node for __leave statements, hook it up.

Codegen is still missing (and I won't work on that), but __leave is now
as implemented as __try and friends.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212425 91177308-0d34-0410-b5e6-96231b3b80d8
Update internals manual on how to add AST nodes.

StmtDumper.cpp is called ASTDumper.cpp these days, and usually works well
for new AST nodes without changes.  There's now DataRecursiveASTVisitor
in addition to RecursiveASTVisitor, and serialization wasn't mentioned.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212426 91177308-0d34-0410-b5e6-96231b3b80d8