[flang] Distinguish intrinsic from non-intrinsic modules

For "USE, INTRINSIC", search only for intrinsic modules;
for "USE, NON_INTRINSIC", do not recognize intrinsic modules.
Allow modules of both kinds with the same name to be used in
the same source file (but not in the same scoping unit, a
constraint of the standard that is now enforced).

The symbol table's scope tree now has a single instance of
a scope with a new kind, IntrinsicModules, whose children are
the USE'd intrinsic modules (explicit or not).  This separate
"top-level" scope is a child of the single global scope and
it allows both intrinsic and non-intrinsic modules of the same
name to exist in the symbol table.  Intrinsic modules' scopes'
symbols now have the INTRINSIC attribute set.

The search path directories need to make a distinction between
regular directories and the one(s) that point(s) to intrinsic
modules.  I allow for multiple intrinsic module directories in
the second search path, although only one is needed today.

Differential Revision: https://reviews.llvm.org/D118631