TypeScript 3.4 RC incremental build path issue

To reproduce failing build

Clone project, npm install, then run:

$ npm run buildFail

Issue description

Build fails when rootDir is passed as CLI flag:

• "incremental": true
• outDir has value (doesn't matter if the value is from tsconfig.json or CLI)
• --rootDir <my path> set from CLI

Fails with this error:

Error: Debug Failure. False expression: Paths must either both be absolute or both be relative
    at Object.getRelativePathFromDirectory (C:\\projects\\node_modules\\typescript\\lib\\tsc.js:12393:15)
    at Object.getOutputPathForBuildInfo (C:\\projects\\node_modules\\typescript\\lib\\tsc.js:67425:55)
    at Object.readBuilderProgram (C:\\projects\\node_modules\\typescript\\lib\\tsc.js:76379:32)
    at performIncrementalCompilation (C:\\projects\\node_modules\\typescript\\lib\\tsc.js:78142:29)
    at Object.executeCommandLine (C:\\projects\\node_modules\\typescript\\lib\\tsc.js:78042:17)
    at Object.<anonymous> (C:\\projects\\node_modules\\typescript\\lib\\tsc.js:78285:4)
    at Module._compile (internal/modules/cjs/loader.js:721:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:732:10)
    at Module.load (internal/modules/cjs/loader.js:620:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:560:12)

Working build

Build works when rootDir is set in tsconfig.json:

• "incremental": true
• outDir has value
• "rootDir": "<my path>" set from tsconfig.json

$ npm run buildWork

Environment information

OS: Windows 10 1809 (17763.379) nodejs version: v11.6.0 npm version: 5.3.0 typescript version: typescript@3.4.0-dev.20190316