[ASTImporter] Set redecl chain of functions before any other import

Summary:
FunctionDecl import starts with a lookup and then we create a new Decl.
Then in case of CXXConstructorDecl we further import other Decls
(base classes, members through CXXConstructorDecl::inits()) before connecting
the redecl chain.  During those in-between imports structural eq fails
because the canonical decl is different.  This commit fixes this.
Synthesizing a test seemed extremely hard, however, Xerces analysis
reproduces the problem.

Reviewers: a_sidorin, a.sidorin

Subscribers: rnkovacs, dkrupp, Szelethus, cfe-commits

Differential Revision: https://reviews.llvm.org/D53702

llvm-svn: 347306