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.

Add validation in the FileList reader to check that the headers exist and use similar diagnostics in Options.cpp

This patch adds support for expected InstallAPI inputs. InstallAPI
accepts a well defined filelist of headers and how those headers
represent a single library.

InstallAPI captures header files to determine linkable symbols to then
compare against what was compiled in a binary dylib and generate TBD
files.