[RISCV] Remove SeenExtMap from RISCVISAInfo::parseArchString. (#97506)

Use the Exts map directly instead of adding to a temporary MapVector
first.

There are a couple functional change from this.
-If an unknown extension is duplicated, we will now print an error for
it being unknown instead of an error for it being duplicated. 
-If an unknown extension is followed by an underscore with no extension after
it, we will error for the unknown extension instead of the dangling
underscore.

These don't seem like serious changes to me. I've updated tests
accordingly.

0 files changed, 0 insertions, 0 deletions