Fix type parameter references when assertions are disabled

[sellimenes]

Fixes #180.

Summary

• Avoid emitting local variable type cache declarations when neither assertions nor warnings are enabled.
• This prevents annotated-only output from referencing generic type parameters outside the t.annotate(... t.function(...)) callback where those parameters are defined.
• Add a regression fixture for a generic function containing let bar: X with assertions disabled.

Tests

• TEST_FILTER='bugs/180-type-parameter-assert-false' yarn --cwd packages/babel-plugin-flow-runtime test
• yarn --cwd packages/babel-plugin-flow-runtime test
• yarn --cwd packages/babel-plugin-flow-runtime lint (exits 0; existing repo-wide jsx-a11y rule warnings remain)
• git diff --check