[flang] Rearrange prototype & code placement of IsCoarray()

A quick fix last week to the shared library build caused
the predicate IsCoarray(const Symbol &) to be moved from
Semantics to Evaluate.  This patch completes that move in
a way that properly combines the existing IsCoarray() tests
for expressions and other object with the test for a symbol.

Differential Revision: https://reviews.llvm.org/D114806