[llvm][llvm-objdump] Fix fatbin handling on 32-bit systems (#141620)

Which fixes a test failure seen on the bots, introduced by
#140286.
```
[ RUN      ] OffloadingBundleTest.checkExtractOffloadBundleFatBinary
ObjectTests: ../llvm/llvm/include/llvm/ADT/StringRef.h:618: StringRef llvm::StringRef::drop_front(size_t) const: Assertion `size() >= N && "Dropping more elements than exist"' failed.
0 0x0a24a990 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/home/tcwg-buildbot/worker/clang-armv8-quick/stage1/unittests/Object/./ObjectTests+0x31a990)
1 0x0a248364 llvm::sys::RunSignalHandlers() (/home/tcwg-buildbot/worker/clang-armv8-quick/stage1/unittests/Object/./ObjectTests+0x318364)
2 0x0a24b410 SignalHandler(int, siginfo_t*, void*) Signals.cpp:0:0
3 0xf46ed6f0 __default_rt_sa_restorer ./signal/../sysdeps/unix/sysv/linux/arm/sigrestorer.S:80:0
4 0xf46ddb06 ./csu/../sysdeps/unix/sysv/linux/arm/libc-do-syscall.S:47:0
5 0xf471d292 __pthread_kill_implementation ./nptl/pthread_kill.c:44:76
6 0xf46ec840 gsignal ./signal/../sysdeps/posix/raise.c:27:6
```
Also reported on 32-bit x86.

I think the cause is the code was casting the result of StringRef.find
into an int64_t. The failure value of find is StringRef::npos, which is
defined as:
static constexpr size_t npos = ~size_t(0);

* size_t(0) is 32 bits of 0s
* the inverse of that is 32 bits of 1s
* Cast to int64_t needs to widen this, and it will preserve the original
value in doing so, which is 0xffffffff.
* The result is 0x00000000ffffffff, which is >= 0, so we keep searching
and try to go off the end of the file.

Or put another way, this equivalent function returns true when compiled
for a 32-bit system:
```
bool fn() {
    size_t foo = ~size_t(0);
    int64_t foo64 = (int64_t)foo;
    return foo64 >= 0;
}
```

Using size_t throughout fixes the problem. Also I don't see a reason it
needs to be a signed number, given that it always searches forward from
the current offset.

Author: DavidSpickett

llvm/lib/Object/OffloadBundle.cpp

@@ -38,12 +38,11 @@ Error extractOffloadBundle(MemoryBufferRef Contents, uint64_t SectionOffset,
                            StringRef FileName,
                            SmallVectorImpl<OffloadBundleFatBin> &Bundles) {
 
-  uint64_t Offset = 0;
-  int64_t NextbundleStart = 0;
+  size_t Offset = 0;
+  size_t NextbundleStart = 0;
 
   // There could be multiple offloading bundles stored at this section.
-  while (NextbundleStart >= 0) {
-
+  while (NextbundleStart != StringRef::npos) {
     std::unique_ptr<MemoryBuffer> Buffer =
         MemoryBuffer::getMemBuffer(Contents.getBuffer().drop_front(Offset), "",
                                    /*RequiresNullTerminator=*/false);
@@ -60,10 +59,9 @@ Error extractOffloadBundle(MemoryBufferRef Contents, uint64_t SectionOffset,
 
     // Find the next bundle by searching for the magic string
     StringRef Str = Buffer->getBuffer();
-    NextbundleStart =
-        (int64_t)Str.find(StringRef("__CLANG_OFFLOAD_BUNDLE__"), 24);
+    NextbundleStart = Str.find(StringRef("__CLANG_OFFLOAD_BUNDLE__"), 24);
 
-    if (NextbundleStart >= 0)
+    if (NextbundleStart != StringRef::npos)
       Offset += NextbundleStart;
   }