Umbrella headers are a concept for Darwin-based libraries. They allow
framework authors to control the order in which their headers should be
parsed and allow clients to access available headers by including a
single header.
InstallAPI will attempt to find the umbrella based on the name of the
framework. Users can also specify this explicitly by using command line
options specifying the umbrella header by file path. There can be an
umbrella header per access level.
This reverts commit b7d8c6188986f62573b9516fe27fdd0c7df1aaf9. And
This reverts commit 2d40f179124f874aca4cf1145fdbc42fb8fb17f3.
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.
