Stop renaming main symbol for WASI

[kateinoigakukun]

Stop renaming main symbol for WASI due to lack of support in the linker

Motivation:

The latest SwiftPM rename main entry point name of executable target to avoid conflicting "main" with test target since swift-tools-version >= 5.5.
The main symbol is renamed to "{{module_name}}_main" and it's renamed again to be "main" when linking the executable target. The former renaming is done by Swift compiler, and the latter is done by linker, so SwiftPM passes some special linker flags for each platform.
But SwiftPM assumed that wasm-ld supports it by returning an empty array instead of nil even though wasm-ld doesn't support it yet.

Modifications:

This patch changed linkerFlagsForRenamingMainFunction to return nil when targeting WASI to indicate it's not supported.

However, for a long-term solution, this renaming feature should be supported on wasm-ld because it's not a limitation of WebAssembly.

See also: swiftwasm/swift#3645
CC: @MaxDesiatov

[MaxDesiatov]

@swift-ci please smoke test

[abertelrud]

Thank you for this fix! I'm actually wondering if it's ever correct for this function to return an empty array. This looks like a logic error from the start. Should it perhaps always return nil if the linker doesn't support it, not just for WASI? This fix is great, but it looks as if on Windows the compiler would still be asked to rename the symbol but the linker wouldn't be asked to name it back.

[kateinoigakukun]

@swift-ci please smoke test

[tomerd]

thanks @kateinoigakukun

[abertelrud]

Thanks @kateinoigakukun! Would it make sense to extend one of the unit tests for this (or add a new one)?

[kateinoigakukun]

@swift-ci please smoke test

[abertelrud]

Thanks for the update, @kateinoigakukun. Having a separate boolean makes sense to me, would be interested in hearing what @neonichu and @tomerd think as well.

[tomerd]

Having a separate boolean makes sense to me, would be interested in hearing what @neonichu and @tomerd think as well.

same: +1, although the current boolean flag is more of canRenameEntrypointFunctionName than shouldRenameEntrypointFunctionName?

[MaxDesiatov]

@swift-ci please smoke test

[abertelrud]

Thanks, this looks good to me!