Fix the module hash stored in -gmodules skeleton units

by using the proper API. The old code produced the address of the hash.  This
fixes a warning printed by LLDB and dsymutil about module hash mismatches and
makes the build build products more deterministic.

rdar://77344315

Author: adrian-prantl

lib/IRGen/IRGenDebugInfo.cpp

@@ -736,9 +736,7 @@ class IRGenDebugInfoImpl : public IRGenDebugInfo {
     // but LLVM detects skeleton CUs by looking for a non-zero DWO id.
     // We use the lower 64 bits for debug info.
     uint64_t Signature =
-        Desc.getSignature()
-            ? (uint64_t)Desc.getSignature()[1] << 32 | Desc.getSignature()[0]
-            : ~1ULL;
+      Desc.getSignature() ? Desc.getSignature().truncatedValue() : ~1ULL;
 
     // Handle Clang modules.
     if (ClangModule) {