[DWARF] Make dwarf::getUnitLengthFieldByteSize() constexpr. NFC.

[ikudrin]

Summary

This will help make some expressions in upcoming patches also constexpr.

[probinson]

LGTM

[dblaikie]

Out of curiosity - is there any particular need/use for this function to be constexpr?

[ikudrin]

In D73036#1834734, @dblaikie wrote:

Out of curiosity - is there any particular need/use for this function to be constexpr?

Yes, this allowed converting some expressions in D71932 and D72900 from static const to constexpr.

llvm/include/llvm/BinaryFormat/Dwarf.h

uint8_t getDwarfOffsetByteSize() const {
     }
     llvm_unreachable("Invalid Format value");
   }
 
   explicit operator bool() const { return Version && AddrSize; }
 };
 
 /// Get the byte size of the unit length field depending on the DWARF format.
-inline uint8_t getUnitLengthFieldByteSize(DwarfFormat Format) {
+constexpr uint8_t getUnitLengthFieldByteSize(DwarfFormat Format) {
   switch (Format) {
   case DwarfFormat::DWARF32:
     return 4;
   case DwarfFormat::DWARF64:
     return 12;
   }
   llvm_unreachable("Invalid Format value");
 }