Auto merge of #39921 - cramertj:add-catch-to-ast, r=nikomatsakis

Add catch {} to AST

Part of #39849. Builds on #39864.

diff --cc src/libsyntax/ast.rs
Simple merge

diff --cc src/libsyntax/fold.rs
Simple merge

diff --cc src/libsyntax/parse/parser.rs
Simple merge