[mlir] fix overflow warning when generating embedded libdevice (llvm#125801)

When building mlir with `-DMLIR_NVVM_EMBED_LIBDEVICE=ON`, there will be
a warning
```
build/tools/mlir/lib/Target/LLVM/libdevice_embedded.c:1: warning: overflow in conversion from ‘int’ to ‘char’ changes value from ‘143’ to ‘-113’ [-Woverflow]
```
which is followed by a large number of characters in stdout.

Fix this to avoid stdout outputting a large number of characters (3e5).

Author: MikaOvO

mlir/lib/Target/LLVM/CMakeLists.txt

@@ -125,7 +125,7 @@ function(embed_binary_to_src file output_file symbol)
     # Convert hex data for C compatibility
     string(REGEX REPLACE "([0-9a-f][0-9a-f])" "0x\\1," filedata ${filedata})
     # Write data to output file
-    file(WRITE ${output_file} "const char ${symbol}[] = {${filedata}};\nconst int ${symbol}_size = sizeof(${symbol});\n")
+    file(WRITE ${output_file} "const unsigned char ${symbol}[] = {${filedata}};\nconst int ${symbol}_size = sizeof(${symbol});\n")
 endfunction()
 
 set(MLIR_NVVM_EMBED_LIBDEVICE 0 CACHE BOOL "Embed CUDA libdevice.bc in the binary at build time instead of looking it up at runtime")

mlir/lib/Target/LLVM/NVVM/Target.cpp

@@ -47,7 +47,7 @@ using namespace mlir::NVVM;
 #define __DEFAULT_CUDATOOLKIT_PATH__ ""
 #endif
 
-extern "C" const char _mlir_embedded_libdevice[];
+extern "C" const unsigned char _mlir_embedded_libdevice[];
 extern "C" const unsigned _mlir_embedded_libdevice_size;
 
 namespace {
@@ -160,7 +160,8 @@ LogicalResult SerializeGPUModuleBase::appendStandardLibs() {
   // Allocate a resource using one of the UnManagedResourceBlob method to wrap
   // the embedded data.
   auto unmanagedBlob = UnmanagedAsmResourceBlob::allocateInferAlign(
-      ArrayRef<char>{_mlir_embedded_libdevice, _mlir_embedded_libdevice_size});
+      ArrayRef<char>{(const char *)_mlir_embedded_libdevice,
+                     _mlir_embedded_libdevice_size});
   librariesToLink.push_back(DenseResourceElementsAttr::get(
       type, resourceManager.insert("_mlir_embedded_libdevice",
                                    std::move(unmanagedBlob))));