bpo-41076: Pre-feed the parser with the f-string expression location (GH-21054)

bpo-41076: Pre-feed the parser with the f-string expression location (GH-21054)

This commit changes the parsing of f-string expressions with the new parser. The parser gets pre-fed with the location of the expression itself (not the f-string, which was what we were doing before). This allows us to completely skip the shifting of the AST nodes after the parsing is completed.