Read/write external resource alignment tag in little-endian

https://reviews.llvm.org/D126446 added support for encoding
binary blobs in MLIR assembly.  To enable cross-architecture
compatibility, these need to be encoded in little-endian format.

This patch is a first step in that direction by reading and
writing the alignment tag that those blobs are prefixed by
in little-endian format.  This fixes assertion failures in
several test cases on big-endian platforms.

The actual content of the blob is not yet handled here.

Differential Revision: https://reviews.llvm.org/D129483

mlir/lib/IR/AsmPrinter.cpp

@@ -2685,8 +2685,9 @@ private:
                    uint32_t dataAlignment) final {
       printFn(key, [&](raw_ostream &os) {
         // Store the blob in a hex string containing the alignment and the data.
+        llvm::support::ulittle32_t dataAlignmentLE(dataAlignment);
         os << "\"0x"
-           << llvm::toHex(StringRef(reinterpret_cast<char *>(&dataAlignment),
+           << llvm::toHex(StringRef(reinterpret_cast<char *>(&dataAlignmentLE),
                                     sizeof(dataAlignment)))
            << llvm::toHex(StringRef(data.data(), data.size())) << "\"";
       });

mlir/lib/Parser/Parser.cpp

@@ -24,6 +24,7 @@
 #include "llvm/ADT/ScopeExit.h"
 #include "llvm/ADT/StringSet.h"
 #include "llvm/ADT/bit.h"
+#include "llvm/Support/Endian.h"
 #include "llvm/Support/PrettyStackTrace.h"
 #include "llvm/Support/SourceMgr.h"
 #include <algorithm>
@@ -2349,7 +2350,7 @@ public:
                          "expected hex string blob for key '" + key +
                              "' to encode alignment in first 4 bytes");
     }
-    uint32_t align = 0;
+    llvm::support::ulittle32_t align;
     memcpy(&align, blobData->data(), sizeof(uint32_t));
 
     // Get the data portion of the blob.