The config path is an optional argument to `ts.parseJsonConfigFileContent`. When passed, it is added to the returned object as `options.configFilePath`, and `tsc` itself passes it in.
The new TS 3.4 [incremental](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-4.html) build functionality relies on this property being present: 025d826339/src/compiler/emitter.ts (L56-L57)
When using The compiler-cli `readConfiguration` the config path option isn't passed, preventing consumers (like @ngtools/webpack) from obtaining a complete config object.
This PR fixes this omission and should allow JIT users of @ngtools/webpack to set the `incremental` option in their tsconfig and have it be used by the TS program.
I tested this in JIT and saw a small decrease in build times in a small project. In AOT the incremental option didn't seem to be used at all, due to how `ngc` uses the TS APIs.
Related to https://github.com/angular/angular-cli/issues/13941.
PR Close#29872
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