TypeChecker sometimes (e.g. property wrappers) inserts implicit decls
(e.g. 'PatternBindingDecl's and 'VarDecl's) between decls in the AST.
This used to confuse 'getEquivalentDeclContextFromSourceFile()'. It
canceled fast-completion, caused crashes, or completed in a wrong
context.
Ignore implicit decls in the AST so that we can find the correct decl
context.
rdar://problem/58665268