[flang] Fix purity testing for generic calls

The purity or impurity of a call to a generic interface
depends on the attributes of the specific procedure or specific
binding.  Change expression analysis of calls to generic interfaces
to replace the symbol in the parse tree with the specific procedure
or binding; this ensures that later checking for purity in
DO CONCURRENT and other contexts will be accurate.

Remove an "XFAIL" from a test that now passes again with this fix.

Differential Revision: https://reviews.llvm.org/D126150