[NFC] Source maps: Simplify the code and add comments

[kripken]

Almost entirely trivial, except for this part:

-  if (nextDebugLocation.availablePos == 0 &&
-      nextDebugLocation.previousPos <= pos) {
+  if (nextDebugLocation.availablePos == 0) {
      return;

I believe removing the extra check has no effect. Removing it does not change
anything in the test suite, and logically, if we set availablePos to 0 then we
definitely want to return here - we set it to 0 to indicate there is nothing left
to read, which is what the code after it does.

As a result, we can remove the previousPos field entirely.

cc @JesseCodeBones - am I missing something about previousPos? Do you have
an example maybe that shows it is needed?

[codecov]

Codecov Report

Merging #5912 (ecea659) into main (e8adbdd) will decrease coverage by 0.02%.
Report is 4 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #5912      +/-   ##
==========================================
- Coverage   42.63%   42.61%   -0.02%     
==========================================
  Files         484      484              
  Lines       74882    74843      -39     
  Branches    11942    11927      -15     
==========================================
- Hits        31925    31898      -27     
+ Misses      39748    39747       -1     
+ Partials     3209     3198      -11     

Files Changed	Coverage Δ
src/wasm-binary.h	83.60% <ø> (ø)
src/wasm/wasm-binary.cpp	53.08% <100.00%> (-0.10%)	⬇️

... and 13 files with indirect coverage changes

[aheejin]

Thanks! This is much easier to understand. By the way we had a similar data fields before #5504 - Then it was

std::pair<uint32_t, Function::DebugLocation> nextDebugLocation;

And now it is

size_t nextDebugPos;
Function::DebugLocation nextDebugLocation;

Then I wonder what #5504 was for..? Given that #5504 seemed to fix something and this is NFC I guess I've missed something...

[kripken]

Given that #5504 seemed to fix something and this is NFC I guess I've missed something...

Yes, I think some parts of that PR were not needed, which are partially reverted here, unless I am mistaken. But other parts of that PR were good fixes, namely

https://github.com/WebAssembly/binaryen/pull/5504/files#diff-666d70a4550a2ccec6247c639d54231a05108efc40af830403dfbc6527b251f7R2762-R2768

and

https://github.com/WebAssembly/binaryen/pull/5504/files#diff-227e746a6f9adba03a777865b9bd74e49db780106f349eae1aaed0ca3d2eb91eL234-R261