[clang-rename] cleanup: use isWritten

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