Handle Lexing.position <-> Location.t conversions in Preprocessor

[nhuurre]

Stateful alternative to #1310 .

With this include_from locations are cached in Preprocessor and we don't need Location.of_string_opt at all.

Submission Checklist

• [x] Run unit tests
• Documentation
  • [ ] If a user-facing facing change was made, the documentation PR is here:
  • [x] OR, no user-facing changes were made

Release notes

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[codecov[bot]]

Codecov Report

Merging #1320 (62dcb6b) into master (f2a65c8) will decrease coverage by 0.03%. The diff coverage is 100.00%.

:exclamation: Current head 62dcb6b differs from pull request most recent head 234dd5f. Consider uploading reports for the commit 234dd5f to get more accurate results

@@            Coverage Diff             @@
##           master    #1320      +/-   ##
==========================================
- Coverage   88.79%   88.76%   -0.03%     
==========================================
  Files          64       64              
  Lines        9853     9821      -32     
==========================================
- Hits         8749     8718      -31     
+ Misses       1104     1103       -1     

Impacted Files	Coverage Δ
src/middle/Location.ml	95.00% <ø> (-1.16%)	:arrow_down:
src/middle/Location_span.ml	90.00% <ø> (-3.34%)	:arrow_down:
src/frontend/Input_warnings.ml	82.35% <100.00%> (-2.65%)	:arrow_down:
src/frontend/Parse.ml	85.29% <100.00%> (ø)
src/frontend/Preprocessor.ml	96.92% <100.00%> (+0.84%)	:arrow_up: