[release/3.1] Adjust #line directives a bit

- was "Add option to `RazorPageGenerator` for `#line` preprocessor directives (#27765) (#29024)"
- cherry-picked from d546861 in 'release/5.0' branch
  - was cherry-picked from 1a241fc in 'main' branch

- avoid new CS1504 errors because compiler can't find the ErrorPage.cshtml file
  - problem seems specific to local builds of this branch
- no tooling changes because `RazorPageGenerator` is in aspnetcore-tooling for 3.1
- change amounts to running 'release/5.0' or 'main' version of tool in this branch
  - ErrorPage.cshtml files in three branches were identical at time of previous commits
    ('main' version of ErrorPage.cshtml recently for nullability)

Author: dougbu