[NFC][AIX][XCOFF] Fix compile warning on strncpy

GCC warning:
```
In file included from /usr/include/string.h:495,
                 from /usr/include/c++/9/cstring:42,
                 from /llvm-project/llvm/include/llvm/ADT/Hashing.h:53,
                 from /llvm-project/llvm/include/llvm/ADT/ArrayRef.h:12,
                 from /llvm-project/llvm/include/llvm/MC/MCAsmBackend.h:12,
                 from /llvm-project/llvm/lib/MC/XCOFFObjectWriter.cpp:14:
In function ‘char* strncpy(char*, const char*, size_t)’,
    inlined from ‘{anonymous}::Section::Section(const char*, llvm::XCOFF::SectionTypeFlags, bool, {anonymous}::CsectGroups)’ at /llvm-project/llvm/lib/MC/XCOFFObjectWriter.cpp:146:12:
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:106:34: warning: ‘char* __builtin_strncpy(char*, const char*, long unsigned int)’ specified bound 8 equals destination size [-Wstringop-truncation]
  106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
      |          ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
         ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

Reviewed By: hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D94872

Author: nullptr-cpp

llvm/lib/MC/XCOFFObjectWriter.cpp

@@ -138,12 +138,13 @@ struct Section {
       Group->clear();
   }
 
-  Section(const char *N, XCOFF::SectionTypeFlags Flags, bool IsVirtual,
+  Section(StringRef N, XCOFF::SectionTypeFlags Flags, bool IsVirtual,
           CsectGroups Groups)
-      : Address(0), Size(0), FileOffsetToData(0), FileOffsetToRelocations(0),
-        RelocationCount(0), Flags(Flags), Index(UninitializedIndex),
-        IsVirtual(IsVirtual), Groups(Groups) {
-    strncpy(Name, N, XCOFF::NameSize);
+      : Name(), Address(0), Size(0), FileOffsetToData(0),
+        FileOffsetToRelocations(0), RelocationCount(0), Flags(Flags),
+        Index(UninitializedIndex), IsVirtual(IsVirtual), Groups(Groups) {
+    assert(N.size() <= XCOFF::NameSize && "section name too long");
+    memcpy(Name, N.data(), N.size());
   }
 };