[lld-macho] Fix Defined size increase with -mms-bitfields (#107545)

Under the Microsoft ABI, only those bit fields can be merged whose
underlying types have the same size.

d175616 (`[lld-macho][arm64] Enhance safe ICF with thunk-based
deduplication`) added an enum field (`identicalCodeFoldingKind`) next to
booleans in the `Defined` class, which increased the size under the MS
ABI. On MinGW targets, this triggered the `static_assert` which checks
the size of `Defined` (for MSVC targets, the check is disabled due to
another problem). Let's store it as a `uint8_t` to allow merging to take
place.

Fixes #107511

Author: BertalanD

lld/MachO/Symbols.h

@@ -14,6 +14,7 @@
 #include "Target.h"
 
 #include "llvm/Object/Archive.h"
+#include "llvm/Support/Compiler.h"
 #include "llvm/Support/MathExtras.h"
 
 namespace lld {
@@ -152,7 +153,8 @@ class Defined : public Symbol {
   // Whether this symbol should appear in the output symbol table.
   bool includeInSymtab : 1;
   // The ICF folding kind of this symbol: None / Body / Thunk.
-  ICFFoldKind identicalCodeFoldingKind : 2;
+  LLVM_PREFERRED_TYPE(ICFFoldKind)
+  uint8_t identicalCodeFoldingKind : 2;
   // Symbols marked referencedDynamically won't be removed from the output's
   // symbol table by tools like strip. In theory, this could be set on arbitrary
   // symbols in input object files. In practice, it's used solely for the