While I am at it, this patch uses const l-value references for
std::shared_ptr.  We don't need to increment the reference count by
passing std::shared_ptr by value.

Identified with llvm-use-ranges.

These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

RPaths are basically search paths for how to load dependent libraries.
The order they appear is the order the linker will search, we should
preserve that order in tbd files.

* Additionally add this level of detection to llvm-readtapi.

resolves: rdar://145603347

This would lead to a malformed TBD file.

It matches up with other _attribute_ adding member functions and helps
simplify InterfaceFile assignment for InstallAPI.

Create a helper method for this operation, so it can be reused in
multiple places.
Additonally move FileType enum into its own header to avoid include
cycles.

An empty string attribute value (e.g. a parent-umbrella: "") is
equivalent to omitting it. Theres no reason to write it out.

… format.

Formats >= TBDv4 will now encode new attribute that the system static
linker wil read when tbd files replace binary dylibs.