* Have clang always append & pass System/Library/SubFrameworks when determining default sdk search paths.
* Teach clang-installapi to traverse there for framework input.
* Teach llvm-readtapi that the library files (TBD or binary) in there should be considered private.

resolves: rdar://137457006

Projects like libc use mutually exclusive macros to compile files
multiple times and then merge the result into the final library. For
installapi to accept these, we'd need to parse the same declarations in
different ways. This patch adds the basic pipelining for installapi to
create the correct TBD file.

* -Xproject allows: -fmodules, -fobjc-arc, fvisibility=hidden, prefix
headers
* -Xlabel allows: -D and -U settings
* Error on 'private' and 'public' labels -X<label>
* Xplatform allows: -iframework <path> This is to support the case where
zippered frameworks want to pass in iOSSupport search path.

Apple's ld supports alias_lists, described as
```
     -alias_list filename
             The specified filename contains a list of aliases. The symbol name and its alias are on one
             line, separated by whitespace.  Lines starting with # are ignored.
```
To handle this for installapi-produced TBD files, pass along the same
input and account for it in verification.

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.

When given a directory input `llvm-readtapi` traverses through the
directory to find dylibs or tbd files to operate on. TBD files will be
created with the same base file name as the dylib. Symlinks should be
created if the input is one.

This also introduces options to delete input files which are defined as
library files that existed before `readtapi -stubify` was invoked. Also
the ability to delete private libraries where private libraries are in a
predefined file system locations on darwin based platforms.