As of CMake 3.25, there are now global variables `LINUX=1`, `ANDROID=1`,
etc. These conflict with expressions that used these names as unquoted
strings in positions where CMake accepts 'variable|string', for example:
- `if(sdk STREQUAL LINUX)` would fail, because `LINUX` is now defined and
expands to 1, where it would previously coerce to a string.
- `if(${sdk} STREQUAL "LINUX")` would fail if `sdk=LINUX`, because the
left-hand side expands twice.
In this patch, I looked for a number of patterns to fix up, sometimes a
little defensively:
- Quoted right-hand side of `STREQUAL` where I was confident it was
intended to be a string literal.
- Removed manual variable expansion on left-hand side of `STREQUAL`,
`MATCHES` and `IN_LIST` where I was confident it was unintended.
Fixes#65028.
`.swiftmodule` is listed as an output of pure swift libraries, but it
isn't updated if there's been no changes (just like the library and
its object files). Add it to the touch hack as well.
Swift *executables* also list `.swiftmodule` as an output, even though
one isn't generated at all. Add a new POST_BUILD command to touch it
regardless.
* Move `add_pure_swift_host_library()` from lib/CMakeLists.txt to
AddPureSwift.cmake so that code outside `lib` can use it
* Add `add_pure_swift_host_tool()` function to make a pure Swift
host executable target (for future usages)
* Specify depending `SwiftSyntax` modules explicitly because not all
Swift libraries uses all SwiftSyntax libraries
