Support more fine-grained source location tracking and use it for functions

[loganfsmyth]

This is a fix for https://phabricator.babeljs.io/T7258 / https://phabricator.babeljs.io/T7255 so @kpdecker I'd love your thoughts if you have a minute.

Basically, this uses the call stack and scoping to track the current ast node's source location as the code is being generated. As code is generated, Babel will read the currently active source location, and create a mapping for the content that is being generated. This allows us to have more direct control over the maps that are generated. We can use this control to clear the source location entirely, or to set a new location part-way through rendering.

In this case, I'm using the ability to change the location to point to the end of the function when we generate the last line of a function. This allows us to solve https://phabricator.babeljs.io/T7258.

[codecov-io]

Current coverage is 85.81%

Merging #3463 into master will decrease coverage by -0.01% as of d065a40

@@            master   #3463   diff @@
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  Files          197     197       
  Stmts        12171   12186    +15
  Branches      2502    2507     +5
  Methods          0       0       
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+ Hit          10446   10458    +12
- Partial        519     522     +3
  Missed        1206    1206       

Review entire Coverage Diff as of d065a40

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[kpdecker]

@loganfsmyth I'm going to try to make time this weekend to test this branch in my project to see if it impacts the negative behavior that I saw. Will report back here.

[loganfsmyth]

@kpdecker Awesome, thanks for taking a look!

[kpdecker]

I believe that I was able to test these changes and they don't regress my particular case. For sanity check, my general process was:

1. Checkout this branch, build with make bootstrap; make build; cd packages/babel-generator; npm link
2. In project npm link babel-generator
3. Purposefully fail a test
4. Examine stack trace to verify that the expect calls are on the call line, not the import line.

When running with both the linked copy and a fresh install of babel-generator@6.7.5, the stack traces were the same.

I don't really have the time review the changes, but no complaints from me on this PR.