[modules] Rationalize the behavior of Decl::declarationReplaces, and in
particular don't assume that two declarations of the same kind in the same
context are declaring the same entity. That's not tr

[modules] Rationalize the behavior of Decl::declarationReplaces, and in
particular don't assume that two declarations of the same kind in the same
context are declaring the same entity. That's not true when the same name is
declared multiple times as internal-linkage symbols within a module.
(getCanonicalDecl is cheap now, so we can just use it here.)

llvm-svn: 251898

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