Set vendor name for --version command

[kateinoigakukun]

We should set vendor name to distinguish our toolchain from official one.

https://github.com/apple/swift/blob/f190342b617f42f2ae0248dfd51233c36d6db773/lib/Basic/Version.cpp#L399-L401

Expected output:

$ swift --version
SwiftWasm Swift version 5.3-dev (LLVM ba56ef042e, Swift 5855a96018)
Target: x86_64-apple-darwin19.3.0

[MaxDesiatov]

How would we also make it say 5.3.0 instead of 5.3-dev when we tag that release? I thought this was included in SWIFT_USER_VISIBLE_VERSION, but the -dev part isn't there.

[kateinoigakukun]

I think if we set SWIFT_COMPILER_VERSION, the -dev suffix will be disappeared.

https://github.com/apple/swift/blob/f190342b617f42f2ae0248dfd51233c36d6db773/lib/Basic/Version.cpp#L405