[LLD] [COFF] Preserve directives and export names from LTO objects (#78802)

The export names are saved as StringRefs pointing into the COFF
directives. In the case of LTO objects, this can be memory allocated
that is owned by the LTO InputFile, which gets destructed when doing the
compilation.

In the case of LTO objects from an older version of LLVM, which require
being upgraded when loaded, the directives string gets destructed, while
when using LTO objects of a matching version (the common case), the
directives string points into memory that doesn't get destructed on LTO
compilation.

Test this by linking a bundled binary LTO object file, from an older
version of LLVM.

This fixes issue #78591, and downstream issue
https://github.com/mstorsjo/llvm-mingw/issues/392.