Update for Swift 5.2

[mattt]

Related to SwiftDocOrg/SwiftSemantics#3

This update also fixes a race condition / heap corruption issue in the SwiftSyntax 0.501000.0 release. With that fixed, we can now safely parallelize the process of parsing source files. Testing locally on our fork of Alamofire, the Swift 5.2 branch is 2.3x faster than the current master.

$ git co master
$ swift build
$ time swift run swift-doc ../Alamofire/Source/
        3.21 real         2.50 user         0.21 sys

$ git co swift-5.2
$ swift build
$ time swift run swift-doc ../Alamofire/Source/
        1.36 real         3.44 user         0.20 sys