* std.cache_hash exposes Hasher type
* std.cache_hash makes hasher_init a global const
* std.cache_hash supports cloning so that clones can share the same
open manifest dir handle as well as fork from shared hasher state
* start to populate the cache_hash for stage2 builds
* remove a footgun from std.cache_hash add function
* get rid of std.Target.ObjectFormat.unknown
* rework stage2 logic for resolving output artifact names by adding
object_format as an optional parameter to std.zig.binNameAlloc
* support -Denable-llvm in stage2 tests
* Module supports the use case when there are no .zig files
* introduce c_object_table and failed_c_objects to Module
* propagate many new kinds of data from CLI into Module and into
linker.Options
* introduce -fLLVM, -fLLD, -fClang and their -fno- counterparts.
closes#6251.
- add logic for choosing when to use LLD or zig's self-hosted linker
* stub code for implementing invoking Clang to build C objects
* add -femit-h, -femit-h=foo, and -fno-emit-h CLI options
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