[Coroutines] Avoid assert splitting hidden coros

Summary:
When attempting to split a coroutine with 'hidden' visibility (for
example, a C++ coroutine that is inlined when compiled with the option
'-fvisibility-inlines-hidden'), LLVM would hit an assertion in
include/llvm/IR/GlobalValue.h:240: "local linkage requires default
visibility". The issue is that the visibility is copied from the source
of the function split in the `CloneFunctionInto` function, but the linkage
is not. To fix, create the new function first with external linkage,
then copy the linkage from the original function *after* `CloneFunctionInto`
is called.

Since `GlobalValue::setLinkage` in turn calls `maybeSetDsoLocal`, the
explicit call to `setDSOLocal` can be removed in CoroSplit.cpp.

Test Plan: check-llvm

Reviewers: GorNishanov, lewissbaker, EricWF, majnemer, rnk

Reviewed By: rnk

Subscribers: llvm-commits, eric_niebler

Differential Revision: https://reviews.llvm.org/D44185

llvm-svn: 329033