[Chapel Merge] Enable the "if-var" construct for conditionals

Branch: refs/heads/master
Revision: fe36965
Author: vasslitvinov
Log Message:

Merge pull request #17047 from vasslitvinov/if-var

Enable the “if-var” construct for conditionals

Implements Strategy (1b) in #13639.

Allows the following syntax:

if const X = someExpression() {   // must be a class
  .... // X is guaranteed to be non-nil
       // X is a borrow, except it is unmanaged if someExpression() is

as well as the following variants:

  • var X instead of const X makes X modifiable within the then-block
  • then keyword instead of {} and else-clauses are allowed as usual

Initially this PR contained applications of this construct in a couple of places in the modules.
I took them out for now and will add them back in later by reverting 391973eae0

r: @mppf

Modified Files:
A test/classes/nilability/if-var-1.chpl
A test/classes/nilability/if-var-1.good
A test/classes/nilability/if-var-2-const.chpl
A test/classes/nilability/if-var-2-const.good
A test/classes/nilability/if-var-2-int.chpl
A test/classes/nilability/if-var-2-int.good
A test/classes/nilability/if-var-2-scope.chpl
A test/classes/nilability/if-var-2-scope.good
A test/classes/nilability/if-var-2-type.chpl
A test/classes/nilability/if-var-2-type.good
M compiler/AST/build.cpp
M compiler/AST/primitive.cpp
M compiler/include/bison-chapel.h
M compiler/include/build.h
M compiler/include/flex-chapel.h
M compiler/include/primitive_list.h
M compiler/parser/bison-chapel.cpp
M compiler/parser/chapel.ypp
M compiler/parser/flex-chapel.cpp
M compiler/passes/normalize.cpp
M compiler/passes/scopeResolve.cpp
M modules/internal/ChapelBase.chpl

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