Fix -commonjs compilation crash and empty output (#1990)

[tangtaizong666]

Summary

Fixes #1990.

hermes -commonjs segfaulted on any input. The crash is a null dereference in genBasicFunction: its enqueued compilation lambda captures curFunction()->typedClassContext / legacyClassContext at enqueue time, and doCJSModule was the only genBasicFunction entry point that did not establish an active FunctionContext first (doIt, doItInScope and doLazyFunction all do). In a release build curFunction() returns the null functionContext_, which matches the ASan trace in the issue (ESTreeIRGen-func.cpp:390:43, READ at offset 0x148). The fix creates a top-level FunctionContext, mirroring doLazyFunction.

With the crash fixed, -commonjs still silently produced empty bytecode (CommonJS module count: 0, nothing executed) at the default optimization level, because two optimizer passes do not treat CJS module functions as roots — they have no IR users, they are only referenced from the module's CJS module table and called by the runtime's require machinery:

• deleteUnusedFunctionsAndVariables deleted them as unused (on main, DCE explicitly skips M->findCJSModule(&F); that check was lost in this helper on static_h).
• analyzeFunctionCallsites marked them allCallsitesKnownInStrictMode/unreachable and their bodies were gutted, producing a segfault at -O; only the global function was exempted as "called by the runtime".

FuncSigOpts already special-cases CJS module functions (F.isGlobalScope() || M->findCJSModule(&F)), so this brings the other two passes in line with it. These are the only two isGlobalScope() special cases in lib/Optimizer without the CJS check.

Test Plan

New regression test test/hermes/cjs-module.js compiles and runs a CommonJS module at the default optimization level, -O0 and -O. TDD verified against each fix:

• Unpatched: hermes -commonjs test.js → Segmentation fault (exit 139) at any opt level.
• With only the IRGen fix: -O0 runs correctly, but default/-O produce no output (module deleted as unused), and after also fixing deleteUnusedFunctionsAndVariables, -O segfaults on the gutted unreachable body — each optimizer fix addresses an observed failure mode.
• With all three fixes:

$ hermes -commonjs test/hermes/cjs-module.js     # same with -O0 / -O
cjs module start
object function object
42

Multi-module require() round trip (also with -O and -O -fstatic-require):

$ hermes -commonjs main.js dep.js
hello world
$ hermes -commonjs -dump-bytecode main.js dep.js | grep 'CommonJS module count'
  CommonJS module count: 2

lit suites for the affected components (test/hermes, test/IRGen, test/Optimizer, test/BCGen, test/Sema, test/AST, test/Parser, release Linux build): 1046 passes, 0 failures caused by this change (the 640 pre-existing failures in my environment are all missing shermes/hbcdump binaries, verified by grepping every failure log).

Code formatted with clang-format (no diff).

[tmikov]

Hmm. CommonJS isn't supported or maintained, it is just we haven't deleted it yet. But this is a small change that fixes something that does exist. We will discuss whether we want to take it. Thanks!