[mach-o] Add support for using export tries

On Darwin at runtime, dyld will prefer to use the export trie of a dylib instead
of the traditional symbol table (which is large and requires a binary search).

This change enables the linker to generate an export trie and to prefer it if
found in a dylib being linked against.  This also simples the yaml for dylibs
because the yaml form of the trie can be reduced to just a sequence of names.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@217066 91177308-0d34-0410-b5e6-96231b3b80d8