[ThinLTO] Respect ClearDSOLocalOnDeclarations for unimported functions

D74751 added `ClearDSOLocalOnDeclarations` and dropped dso_local for
isDeclarationForLinker `GlobalValue`s. It missed a case for imported
declarations (`doImportAsDefinition` is false while `isPerformingImport` is
true). This can lead to a linker error for a default visibility symbol in
`ld.lld -shared`.

When `ClearDSOLocalOnDeclarations` is true, we check
`isPerformingImport() && !doImportAsDefinition(&GV)` along with
`GV.isDeclarationForLinker()`. The new condition checks an imported declaration.

This patch fixes a `LLVMPolly.so` link error using a trunk clang -DLLVM_ENABLE_LTO=Thin.

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D104986