[flang][NFC] Add GetTopLevelUnitContaining functions

`GetTopLevelUnitContaining` returns the Scope nested in the global scope
that contains the given Scope or Symbol.

Use "Get" rather than "Find" in the name because "Find" implies it might
not be found, which can't happen. Following that logic, rename
`FindProgramUnitContaining` to `GetProgramUnitContaining` and have it
also return a reference rather that a pointer.

Note that the use of "ProgramUnit" is slightly confusing. In the Fortran
standard, "program-unit" refers to what is called a "TopLevelUnit" here.
What we are calling a "ProgramUnit" (here and in `ProgramTree`) includes
internal subprograms while "TopLevelUnit" does not.

Differential Revision: https://reviews.llvm.org/D92491