[clang] fixing conditional explicit for out-of-line definition PR42980

Summary: not every read in CXXConstructorDecl::getExplicitSpecifierInternal() was made on the canonical declaration.

Reviewers: rsmith, aaron.ballman

Reviewed By: rsmith

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67889

llvm-svn: 372530