[DWARF] Make dwarf::getDwarfOffsetByteSize() a free function. NFC.

[ikudrin]

Summary

This will help simplify code in upcoming patches and make some expressions constexpr.

[aprantl]

llvm/include/llvm/BinaryFormat/Dwarf.h

543

Cool. I hadn't realized that llvm_unreachable is constexpr.

llvm/include/llvm/BinaryFormat/Dwarf.h

 unsigned FormVendor(Form F);
 unsigned OperationVendor(LocationAtom O);
 unsigned AttributeEncodingVendor(TypeKind E);
 unsigned LanguageVendor(SourceLanguage L);
 /// @}
 
 Optional<unsigned> LanguageLowerBound(SourceLanguage L);
 
+/// The size of a reference determined by the DWARF 32/64-bit format.
+constexpr uint8_t getDwarfOffsetByteSize(DwarfFormat Format) {
+  switch (Format) {
+  case DwarfFormat::DWARF32:
+    return 4;
+  case DwarfFormat::DWARF64:
+    return 8;
+  }
+  llvm_unreachable("Invalid Format value");
+}
+
 /// A helper struct providing information about the byte size of DW_FORM
 /// values that vary in size depending on the DWARF version, address byte
 /// size, or DWARF32/DWARF64.
 struct FormParams {
   uint16_t Version;
   uint8_t AddrSize;
   DwarfFormat Format;
 
   /// The definition of the size of form DW_FORM_ref_addr depends on the
   /// version. In DWARF v2 it's the size of an address; after that, it's the
   /// size of a reference.
   uint8_t getRefAddrByteSize() const {
     if (Version == 2)
       return AddrSize;
     return getDwarfOffsetByteSize();
   }
 
   /// The size of a reference is determined by the DWARF 32/64-bit format.
   uint8_t getDwarfOffsetByteSize() const {
-    switch (Format) {
-    case DwarfFormat::DWARF32:
-      return 4;
-    case DwarfFormat::DWARF64:
-      return 8;
-    }
-    llvm_unreachable("Invalid Format value");
+    return dwarf::getDwarfOffsetByteSize(Format);
   }
 
   explicit operator bool() const { return Version && AddrSize; }
 };
 
 /// Get the byte size of the unit length field depending on the DWARF format.
 constexpr uint8_t getUnitLengthFieldByteSize(DwarfFormat Format) {
   switch (Format) {