nit: use isWritten and const auto *Initializer in
NamedDeclFindingASTVisitor::VisitCXXConstructorDecl method.
Test plan: make -j8 check-clang-tools (passed)
Patch by Alexander Shaposhnikov!
Reviewers: omtcyfz
Differential Revision: https://reviews.llvm.org/D23298
llvm-svn: 278112