Ideally this would also update the `--version` output to be overridden
by `SWIFT_TOOLCHAIN_VERSION`, but unfortunately various tools rely on
the current format (eg. swift-build).
To work-around #80059, we need to stop return address signing and
opt-out of BTCFI enforcement via enabling a platform linker option.
We don't want to completely undo the BTCFI work in the rare case that
we later figure out how to properly address the above issue, or allow
users who might want to benefit from BTCFI enforcement and won't use
Concurrency. To do this, condition the existing BTCFI flag enforcement
into a configuration option that defaults to off for now.
Because the new swift-driver needs to "know" whether the frontend is
configured to opt-out or not, and since the new driver communicates with
the frontend via the target info JSON to begin with, we add a field
that emits the build flavor to signal the right behavior.
Use the `clang::TargetInfo` to query the target pointer size for the
given triple. This is meant to enable us to properly determine
`CMAKE_SIZEOF_VOID_P`.
When looking for a Swift module on disk, we were scanning all module search paths if they contain the module we are searching for. In a setup where each module is contained in its own framework search path, this scaled quadratically with the number of modules being imported. E.g. a setup with 100 modules being imported form 100 module search paths could cause on the order of 10,000 checks of `FileSystem::exists`. While these checks are fairly fast (~10µs), they add up to ~100ms.
To improve this, perform a first scan of all module search paths and list the files they contain. From this, create a lookup map that maps filenames to the search paths they can be found in. E.g. for
```
searchPath1/
Module1.framework
searchPath2/
Module1.framework
Module2.swiftmodule
```
we create the following lookup table
```
Module1.framework -> [searchPath1, searchPath2]
Module2.swiftmodule -> [searchPath2]
```
