JS: Remove synthetic locations

[asgerf]

For background see #20297.

This PR removes synthetic locations because the dependencies induced by such locations is greatly complicating overlay locality. This might change the location associated with some SSA phi nodes (from the legacy SSA library), but such nodes aren't generally used for user-visible alerts anyway.

Evaluation looks good. There's a 1% speed-up, which is expected given that we saw a similar slow-down when synthetic locations were introduced (which was at the time necessary for replacing hasLocationInfo with getLocation).

[Copilot]

Pull Request Overview

This PR removes synthetic locations from the JavaScript CodeQL library to simplify overlay locality handling. The change affects how SSA phi nodes are located but should not impact user-visible alerts since these nodes are typically not shown to users.

• Eliminates the entire synthetic location system and related TLocation type
• Updates all location references from DbLocation to Location throughout the codebase
• Simplifies SSA definition location handling to use basic block locations instead of synthetic ones

Reviewed Changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
javascript/ql/lib/semmle/javascript/internal/Locations.qll	Complete removal of synthetic location implementation and TLocation type
javascript/ql/lib/semmle/javascript/Locations.qll	Simplifies Location class and deprecates DbLocation
javascript/ql/lib/semmle/javascript/SSA.qll	Updates SSA definitions to use basic block locations and deprecates hasLocationInfo
javascript/ql/lib/semmle/javascript/dataflow/internal/sharedlib/Ssa.qll	Changes type parameter from DbLocation to Location
javascript/ql/lib/semmle/javascript/dataflow/internal/VariableOrThis.qll	Updates return types from DbLocation to Location
javascript/ql/lib/semmle/javascript/dataflow/internal/VariableCapture.qll	Updates type parameters and return types to use Location
javascript/ql/lib/semmle/javascript/Variables.qll	Changes LocalVariable.getLocation return type
javascript/ql/lib/semmle/javascript/XML.qll	Updates XML module to use Location instead of DbLocation
javascript/ql/lib/semmle/javascript/YAML.qll	Removes DbLocation type constraint
javascript/ql/lib/semmle/javascript/RestrictedLocations.qll	Updates location reference in FirstLineOf
javascript/ql/lib/semmle/javascript/JSON.qll	Updates JSON file location handling
javascript/ql/lib/semmle/javascript/Files.qll	Updates File.getLocation return type
javascript/ql/lib/semmle/javascript/AST.qll	Updates token location handling

[aschackmull]

LGTM 🎉

[asgerf]

Turns out some locations were negatively affected after all. Taking back into a draft while I fix these.

[asgerf]

There was an issue with SsaExplicitDefinition getting a worse location than before. Previously they were associated with the CFG node of an assignement, which actually pretty annoying for simultaneous assignments. This was accidentally changed to take its location from the start of the basic block of that assignment.

I've updated it to refer to what it should have been all along, the mention of the variable on the LHS. See example of the old and new location:

Old location

New location

Evaluation

Evaluation is not quite 1% anymore, but still leaning in the right direction,