[llvm] [IR] Allow MDString in operand bundles (PR #110805)

[via llvm-commits]

R-Goc wrote:

This commit causes a warning during build time:
```
C:\Users\rysza\lib\llvm-project\llvm\lib\Bitcode\Reader\BitcodeReader.cpp(801,15): warning: comparison of integers of different signs: 'unsigned int' and 'llvm::bitc::MetadataOperandBundleValueMarker' [-Wsign-compare]
  801 |     if (ValID != bitc::OB_METADATA) {
      |         ~~~~~ ^  ~~~~~~~~~~~~~~~~~
```


https://github.com/llvm/llvm-project/pull/110805