Swift: replace internal swift mangler with our own

Our mangler is split in two version:
* `SwiftTrapMangler`, with the same behaviour as the previous
  `SwiftMangler`, constructing mangled names with trap label references
* `SwiftRecursiveMangler` that replaces trap label references with
  recursive calls to its own `mangle` functions, effectively rolling out
  the entire chain of references

The latter is used to create lazy trap file names. Hashing is used to
avoid excessively long filenames.

swift/extractor/infra/SwiftMangledName.cpp

@@ -1,8 +1,17 @@
 #include "swift/extractor/infra/SwiftMangledName.h"
 #include "absl/strings/str_cat.h"
 
+#include <picosha2.h>
+
 namespace codeql {
 
+std::string SwiftMangledName::hash() const {
+  auto ret = picosha2::hash256_hex_string(value);
+  // half a hash is already enough for disambuiguation
+  ret.resize(ret.size() / 2);
+  return ret;
+}
+
 SwiftMangledName& SwiftMangledName::operator<<(UntypedTrapLabel label) & {
   absl::StrAppend(&value, "{", label.str(), "}");
   return *this;