This reverts commit b7d8c61889. And
This reverts commit 2d40f17912.
It caused a build failure i'll need to reproduce.
` error: could not convert ‘Rule’ from ‘llvm::Regex’ to ‘llvm::Expected<llvm::Regex>’`
InstallAPI takes a json list of headers that is typically generated from
a build system like Xcode based on a project's attributes. Sometimes,
maintainers may want to alter this for tapi input. Using e.g.
`--extra-public-headers`, users can manipulate what headers will be used
for TBD file generation.