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

This will help make some expressions in upcoming patches constexpr.

Differential Revision: https://reviews.llvm.org/D73036

Author: igorkudrin

llvm/include/llvm/BinaryFormat/Dwarf.h

@@ -564,7 +564,7 @@ struct FormParams {
 };
 
 /// 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;