[clang] Fix crash with -funique-internal-linkage-names

Calling `getFunctionLinkage(CalleeInfo.getCalleeDecl())` will crash when the declaration does not have a body, e.g., `extern void foo();`. Instead, we can use `isExternallyVisible()` to see if the delcaration has internal linkage.

I believe using `!isExternallyVisible()` is correct because the clang linkage must be `InternalLinkage` or `UniqueExternalLinkage`, both of which are "internal linkage" in llvm.
https://github.com/llvm/llvm-project/blob/9c26f51f5e178ac0fda98419e3a61d205d3b58b1/clang/include/clang/Basic/Linkage.h#L28-L40

Fixes https://github.com/llvm/llvm-project/issues/54139

Reviewed By: tmsriram

Differential Revision: https://reviews.llvm.org/D135926