[ORC] Refactor visit-members  in StaticLibraryDefinitionGenerator. (#141546)

This refactor was motivated by two bugs identified in out-of-tree
builds:

1. Some implementations of the VisitMembersFunction type (often used to	
implement special loading semantics, e.g. -all_load or -ObjC) were assuming
that buffers for archive members were null-terminated, which they are not in
general. This was triggering occasional assertions.

2. Archives may include multiple members with the same file name, e.g.
when constructed by appending files with the same name:
  % llvm-ar crs libfoo.a foo.o
  % llvm-ar q libfoo.a foo.o
  % llvm-ar t libfoo.a foo.o
  foo.o

   While confusing, these members may be safe to link (provided that they're
   individually valid and don't define duplicate symbols). In ORC however, the
   archive member name may be used to construct an ORC initializer symbol,
   which must also be unique. In that case the duplicate member names lead to a
   duplicate definition error even if the members define unrelated symbols.

In addition to these bugs, StaticLibraryDefinitionGenerator had grown a
collection of all member buffers (ObjectFilesMap), a BumpPtrAllocator
that was redundantly storing synthesized archive member names (these are
copied into the MemoryBuffers created for each Object, but were never
freed in the allocator), and a set of COFF-specific import files.

To fix the bugs above and simplify StaticLibraryDefinitionGenerator this
patch makes the following changes:

1. StaticLibraryDefinitionGenerator::VisitMembersFunction is generalized
   to take a reference to the containing archive, and the index of the
   member within the archive. It now returns an Expected<bool> indicating
   whether the member visited should be treated as loadable, not loadable,
   or as invalidating the entire archive.
2. A static StaticLibraryDefinitionGenerator::createMemberBuffer method
   is added which creates MemoryBuffers with unique names of the form
   `<archive-name>[<index>](<member-name>)`. This defers construction of
   member names until they're loaded, allowing the BumpPtrAllocator (with
   its redundant name storage) to be removed.
3. The ObjectFilesMap (symbol name -> memory-buffer-ref) is replaced
   with a SymbolToMemberIndexMap (symbol name -> index) which should be
   smaller and faster to construct.
4. The 'loadability' result from VisitMemberFunctions is now taken into
   consideration when building the SymbolToMemberIndexMap so that members
   that have already been loaded / filtered out can be skipped, and do not
   take up any ongoing space.
5. The COFF ImportedDynamicLibraries member is moved out into the
   COFFImportFileScanner utility, which can be used as a
   VisitMemberFunction.

This fixes the bugs described above; and should lower memory consumption
slightly, especially for archives with many files and / or symbol where
most files are eventually loaded.