[flang] Changes for constraint C1128.

Specifically, these changes enforce the last sentence of the constraint, which
prohibits names that cannot appear in a variable definition context from
appearing in a locality-spec.  Here are the details.

 - Created the function "IsModifiableName" to return "true" when its parameter
   is the name of a variable that can appear in a variable definition context.
 - Created the function "GetAssociationRoot" to follow construct associations
   to potentially get to an underlying variable.  This function is similar to
   the existing "GetUltimate" function that follows use associations and host
   associations.  One difference is that "GetAssociationRoot" requires access
   to the types "MaybeExpr" and "SomeExpr", which makes is inappropriate to put
   into symbol.cc, which is where "GetUltimate" lives.  Perhaps we should move
   "GetUltimate" to tools.[h,cc].
 - Generalized the functions "IsPureFunction" to "IsPureProcedure" since either
   a pure function or subroutine can provide a context for variables that
   cannot be modified.  Changed "FindPureFunctionContaining" to
   "FindPureProcedueContaining" to go along with this.
 - Added the function "IsExternalInPureContext" to detect the case where a
   nominally pure procedure potentially modifies a variable.
 - Created the function "IsOrContainsEventOrLockComponent" to detect variables
   that either are of EVENT_TYPE or LOCK_TYPE or contain components of these
   types.  Such variables cannot appear in variable definition contexts.
 - Added the test resolve56.f90 to test most of these conditions.  Note that I
   only tested the new code from the perspective of locality-specs.

Original-commit: flang-compiler/f18@c9d2507b74da881dda2eb11805a0394a415db2e4
Reviewed-on: https://github.com/flang-compiler/f18/pull/596
Tree-same-pre-rewrite: false