[dsymutil] Avoid copying swiftinterfaces from the SDK into the dsym bundle

This patch fixes a bug that would cause dsymutil to collect
.swiftinterface files for the Swift stdlib and other SDK
modules. There is no advantage in copying these since they should be
loaded from the ones bundled with LLDB's embedded Swift compiler
instead and copying them will cause LLDB to recompile them from source
instead of loading their prebuilt cached counterparts in the SDK.

rdar://problem/57463247

Differential Revisions: https://reviews.llvm.org/D75196