Our mangler is split in two version:
* `SwiftTrapMangler`, with the same behaviour as the previous
`SwiftMangler`, constructing mangled names with trap label references
* `SwiftRecursiveMangler` that replaces trap label references with
recursive calls to its own `mangle` functions, effectively rolling out
the entire chain of references
The latter is used to create lazy trap file names. Hashing is used to
avoid excessively long filenames.
* visiting now happens in a later stage than fetching labels. While
fetching a list of entities to be visited is created, and then acted
upon in actual extraction. This partially flattens the recursive
nature of `fetchLabel` into a loop inside `SwiftVisitor::extract`.
Recursion in `fetchLabel` will only happen on labels fetched while
naming an entity (calling into `SwiftMangler`).
* The choice whether to name a declaration or type has been moved from
the translators to `SwiftMangler`. Acting on this choice is contained
in `SwiftDispatcher::createLabel`.
* The choice whether to emit a body of a declaration has been moved from
`DeclTranslator` to the dispatcher. This choice is also contained in
`SwiftDispatcher::createLabel`.
* The simple functionality of the `LabelStore` has been moved to the
`SwiftDispatcher` as well.
It turns out mangled names can sometimes be too long. While this code
will eventually be replaced by our own mangling, we need to use hashing
to cut down the names.
Module and decl names are preserved in the trap file names for
debuggability.
WMO stands for whole module optimization. It's a compilation mode where
all sources of a module are compiled together, e.g.
```
swift-frontend -emit-module A.swift B.swift -o Module.swiftmodule
```
This is opposed to incremental mode, where one would do something like
```
swift-frontend -emit-module -primary-file A.swift B.swift -module-name Module -o Module~A.swiftmodule
swift-frontend -emit-module A.swift -primary-file B.swift -module-name Module -o Module~B.swiftmodule
swift-frontend -merge-modules Module~A.swiftmodule Module~B.swiftmodule -o Module.swiftmodule
```
In WMO mode we were skipping extraction of all files after the first
one, because we were filtering in only files with an associated output,
and internally swift only assigns the output to the first input file in
WMO mode (which is just an implementation detail).
This patch refines that filter, by getting all input source files in
case there are no primary inputs.
This replaces usages of `llvm::fs` and string manipulation with
`std::filesystem`, also replacing `std::string` with
`std::filesystem::path` where it made sense.
Moreover MD5 hashing used in macOS file remapping was replaced by
SHA256 hashing using a small header-only SHA256 C++ library with an
MIT license, https://github.com/okdshin/PicoSHA2.
File contents hashing was relocated to the newly created `file` library
for later planned reuse.
