The tentantive parse is used for diagnostic purposes but can cause code-completion to delay the same decl twice.
The range of CodeCompletionExpr was previously character range which invalidated invariants of the AST.
Fixes:
validation-test/IDE/crashers_fixed/084-swift-parser-consumedecl.swift
validation-test/IDE/crashers_fixed/104-swift-gettypeofcompletioncontextexpr.swift