Use the real sourcemap API and handle input sourcemaps - fixes T7259

[loganfsmyth]

This does two things:

1. Use the public .addMappingAPI instead manually accessing ._mappings
2. Pass sourceFileName to set the source value on the sourcemaps generated by Babel, rather than overriding the values.

2 here is a behavior change / new feature depending on how you look at it. It means input sourcemaps will actually work. Before, if you had an input sourcemap, we would basically use the mappings from it, but then discard them and pretend the mapping was actually for the JS file given to Babel to compile, which would produce incorrect final sourcemaps.

The dramatic size reduction in the test output file in this example is actually because of our use of result.actual, because it contained an input sourcemap, and at this stage it was not stripped out. By relying on Babel to set the source name and content, we allow Babel's internal logic to strip off the input sourcemap properly.

[codecov-io]

Current coverage is 85.79%

Merging #3456 into master will decrease coverage by 1.90%

1. 8 files (not in diff) in ...ypes/lib/definitions were modified. more
   • Misses -4
   • Partials +13
   • Hits +8
2. 7 files (not in diff) in ...ages/babel-types/lib were modified. more
   • Misses -41
   • Partials +40
   • Hits -134
3. 3 files (not in diff) in ...l-traverse/lib/scope were modified. more
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   • Partials +26
   • Hits +417
4. 3 files (not in diff) in ...raverse/lib/path/lib were modified. more
   • Misses -1
   • Partials +11
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5. 3 files (not in diff) in ...e/lib/path/inference were modified. more
   • Misses -3
   • Partials +20
   • Hits -21
6. 11 files (not in diff) in ...el-traverse/lib/path were modified. more
   • Misses -22
   • Partials +80
   • Hits -6
7. 5 files (not in diff) in ...s/babel-traverse/lib were modified. more
   • Misses -13
   • Partials +17
   • Hits +55
8. 2 files (not in diff) in ...s/babel-register/lib were modified. more
   • Misses +1
   • Partials +11
   • Hits +2
9. 7 files (not in diff) in ...form-regenerator/lib were modified. more
   • Misses -17
   • Partials +33
   • Hits -29
10. 4 files (not in diff) in ...s2015-parameters/lib were modified. more
    • Misses -3
    • Partials +5
    • Hits +39

@@             master   #3456   diff @@
=======================================
  Files           193     197      +4   
  Lines          9384   12171   +2787   
  Methods        1068       0   -1068   
  Branches       2161    2501    +340   
=======================================
+ Hits           8229   10442   +2213   
- Misses         1155    1206     +51   
- Partials          0     523    +523   

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