swift: 4.2.3 -> 5.0.1

update Swift
* new tweaks to the build process are mostly documented inline
* the swiftc wrapper needs the linker path for libicu
* some older tweaks could be removed
* remove fuzz from patches by updating line numbers
* one build fix to libdispatch has been filed upstream

pkgs/development/compilers/swift/purity.patch

@@ -11,7 +11,7 @@ diff --git a/lib/Driver/ToolChains/Gnu.cpp b/lib/Driver/ToolChains/Gnu.cpp
 index fe3c0191bb..c6a482bece 100644
 --- a/lib/Driver/ToolChains/Gnu.cpp
 +++ b/lib/Driver/ToolChains/Gnu.cpp
-@@ -398,13 +398,6 @@ void tools::gnutools::Linker::ConstructJob(Compilation &C, const JobAction &JA,
+@@ -380,13 +380,6 @@ void tools::gnutools::Linker::ConstructJob(Compilation &C, const JobAction &JA,
    if (!Args.hasArg(options::OPT_static)) {
      if (Args.hasArg(options::OPT_rdynamic))
        CmdArgs.push_back("-export-dynamic");