[Modules] Fix LLVM_ENABLE_MODULES build (NFC) (#164402)

FileOffset.h uses uint64_t but only includes <cstdlib>, which is not
specified to provide the uint64_t type.
This happens to work without LLVM_ENABLE_MODULES enabled due to
transitive includes from <cstdlib>.
With LLVM_ENABLE_MODULES enabled, declarations from transitive
includes are not visible by default, and the build fails.

This fixes LLVM_ENABLE_MODULES builds by including the proper header,
<cstdint>, instead.

Author: naveen-seth

llvm/include/llvm/CAS/FileOffset.h

@@ -15,7 +15,7 @@
 #ifndef LLVM_CAS_FILEOFFSET_H
 #define LLVM_CAS_FILEOFFSET_H
 
-#include <cstdlib>
+#include <cstdint>
 
 namespace llvm::cas {