stage2: introduce CacheMode

The two CacheMode values are `whole` and `incremental`.
`incremental` is what we had before; `whole` is new.
Whole cache mode uses everything as inputs to the cache hash;
and when a hit occurs it skips everything including linking.
This is ideal for when source files change rarely and for backends that
do not have good incremental compilation support, for example
compiler-rt or libc compiled with LLVM with optimizations on.
This is the main motivation for the additional mode, so that we can have
LLVM-optimized compiler-rt/libc builds, without waiting for the LLVM
backend every single time Zig is invoked.

Incremental cache mode hashes only the input file path and a few target
options, intentionally relying on collisions to locate already-existing
build artifacts which can then be incrementally updated.

The bespoke logic for caching stage1 backend build artifacts
is removed since we now have a global caching mechanism for
when we want to cache the entire compilation, *including* linking.
Previously we had to get "creative" with libs.txt and a special
byte in the hash id to communicate flags, so that when the cached
artifacts were re-linked, we had this information from stage1
even though we didn't actually run it. Now that `CacheMode.whole`
includes linking, this extra information does not need to be
preserved for cache hits. So although this changeset introduces
complexity, it also removes complexity.

The main trickiness here comes from the inherent differences between the
two modes: `incremental` wants a directory immediately to operate on,
while `whole` doesn't know the output directory until the compilation is
complete. This commit deals with this problem mostly inside `update()`,
where, on a cache miss, it replaces `zig_cache_artifact_directory` with a
temporary directory, and then renames it into place once the compilation is
complete.

Items remaining before this branch can be merged:

* [ ] make sure these things make it into the cache manifest:
  - @import files
  - @embedFile files
  - we already add dep files from c but make sure the main .c files make
    it in there too, not just the included files

* [ ] double check that the emit paths of other things besides the binary
  are working correctly.

* [ ] test `-fno-emit-bin` + `-fstage1`
* [ ] test `-femit-bin=foo` + `-fstage1`

* [ ] implib emit directory copies bin_file_emit directory in create() and needs
  to be adjusted to be overridden as well.

* [ ] make sure emit-h is handled correctly in the cache hash
* [ ] Cache: detect duplicate files added to the manifest

Some preliminary performance measurements of wall clock time and
peak RSS used:

stage1 behavior (1077 tests), llvm backend, release build:
 * cold global cache: 4.6s, 1.1 GiB
 * warm global cache: 3.4s, 980 MiB

stage2 master branch behavior (575 tests), llvm backend, release build:
 * cold global cache: 0.62s, 191 MiB
 * warm global cache: 0.40s, 128 MiB

stage2 this branch behavior (575 tests), llvm backend, release build:
 * cold global cache: 0.62s, 179 MiB
 * warm global cache: 0.27s, 90 MiB

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