[flang] Use structureComponent.component.symbol directly instead of getting

the symbol from the result of expression analysis.  (This opens the
door to moving structure component semantic checking to expression
analysis.)

Also, improve error messages, remove unused include files, and add
copyright headers to test files.

Original-commit: flang-compiler/f18@17780c40d7380db9760e972dcc52724838cd89d8
Reviewed-on: https://github.com/flang-compiler/f18/pull/388