[dsymutil] Collect parseable Swift interfaces in the .dSYM bundle.

When a Swift module built with debug info imports a library without
debug info from a textual interface, the textual interface is
necessary to reconstruct types defined in the library's interface. By
recording the Swift interface files in DWARF dsymutil can collect them
and LLDB can find them.

This patch teaches dsymutil to look for DW_TAG_imported_modules and
records all references to parseable Swift ingterfrace files and copies
them to

  a.out.dSYM/Contents/Resources/<Arch>/<ModuleName>.swiftinterface

<rdar://problem/49751748>

llvm-svn: 358921