rts/linker: Clarify representation of SymType
Previously SymType was both an enumeration of three symbol types and
an orthogonal flag (DUP_DISCARD
, introduced in !9475 (closed)). This was quite
fragile as it meant that to extract the symbol type one had to careful
mask out the flag. Naturally this wasn't done consistently.
Fix this by renaming the field to flags
and adding an accessor.
Fixes #24117.