[SwiftCompilerSources][build] Require recent Swift compiler in `HOSTTOOLS` mode
[SwiftCompilerSources][build] Require recent Swift compiler in `HOSTTOOLS` mode

Some code in SwiftCompilerSources, especially the parts that use C++ interop, triggers bugs in older versions of the Swift compiler. To make sure that `HOSTTOOLS` builds don't produce obscure compiler errors or crashes, let's require a relatively new Swift compiler for `HOSTTOOLS` build.

benlangmuir/swift,glessard/swift,JGiola/swift,benlangmuir/swift,roambotics/swift,glessard/swift,JGiola/swift,JGiola/swift,roambotics/swift,glessard/swift,benlangmuir/swift,glessard/swift,JGiola/swift,roambotics/swift,apple/swift,apple/swift,apple/swift,roambotics/swift,JGiola/swift,glessard/swift,benlangmuir/swift,roambotics/swift,JGiola/swift,apple/swift,apple/swift,roambotics/swift,benlangmuir/swift,benlangmuir/swift,glessard/swift,apple/swift