A custom `SourceMapSource` can optionally provide its own `skipTrivia`
function. If this is not provided then the compiler will use the default
function designed for TypeScript source files.
Previously, when calling this default function we were passing the current
`sourceMapSource` rather than the specified `source` whose trivia needs
to be skipped. This resulted in the `pos` being incorrectly calculated for
external source files that need mapping.
**Side note:**
There are actually two possible constructors available for creating
`SourceMapSource` objects. One of them defaults to an identity function
for the `skipTrivia` function if it is not provided (see
https://github.com/Microsoft/TypeScript/blob/49689894d747714d6b2b8461b9033020efec9625/src/compiler/utilities.ts#L6972-L6976)
and the other one leaves the `skipTrivia` field `undefined` (see
https://github.com/Microsoft/TypeScript/blob/5fc8f1dd801dbacfe7e2d624f80b7a6a3868d180/src/services/services.ts#L776-L797)
Unfortunately, it appears that the second of these two constructors is the
one available when importing the "typescript" module in node.js code.
