Swift: use hashing for lazy decl trap file names

It turns out mangled names can sometimes be too long. While this code
will eventually be replaced by our own mangling, we need to use hashing
to cut down the names.

Module and decl names are preserved in the trap file names for
debuggability.

swift/extractor/SwiftExtractor.cpp

@@ -14,6 +14,7 @@
 #include "swift/extractor/infra/SwiftLocationExtractor.h"
 #include "swift/extractor/infra/SwiftBodyEmissionStrategy.h"
 #include "swift/extractor/mangler/SwiftMangler.h"
+#include <picosha2.h>
 
 using namespace codeql;
 using namespace std::string_literals;
@@ -50,8 +51,21 @@ static fs::path getFilename(swift::ModuleDecl& module,
     return resolvePath(primaryFile->getFilename());
   }
   if (lazyDeclaration) {
+    // this code will be thrown away in the near future
     SwiftMangler mangler;
-    return mangler.mangledName(*lazyDeclaration);
+    auto mangled = mangler.mangledName(*lazyDeclaration);
+    // mangled name can be too long to use as a file name, so we can't use it directly
+    mangled = picosha2::hash256_hex_string(mangled);
+    std::string ret;
+    ret += module.getRealName().str();
+    ret += '_';
+    llvm::SmallVector<char> scratch;
+    // lazyDeclaration must be a ValueDecl, as already asserted in SwiftMangler::mangledName
+    ret += llvm::cast<swift::ValueDecl>(lazyDeclaration)->getName().getString(scratch);
+    ret += '_';
+    // half a SHA2 is enough
+    ret += std::string_view(mangled).substr(0, mangled.size() / 2);
+    return ret;
   }
   // PCM clang module
   if (module.isNonSwiftModule()) {