Teach llvm_add_library() to find include dirs.

Teach llvm_add_library() to find include dirs.

Since header files are not compilation units, CMake does not require
you to specify them in the CMakeLists.txt file.  As a result, unless a
header file is explicitly added, CMake won't know about it, and when
generating IDE-based projects, CMake won't put the header files into
the IDE project.  LLVM currently tries to deal with this in two ways:

  1) It looks for all .h files that are in the project directory, and
     adds those.
  2) llvm_add_library() understands the ADDITIONAL_HEADERS argument,
     which allows one to list an arbitrary list of headers.

This patch takes things one step further.  It adds the ability for
llvm_add_library() to take an ADDITIONAL_HEADER_DIRS argument, which
will specify a list of folders which CMake will glob for header files.
Furthermore, it will glob not only for .h files, but also for .inc
files.

Included in this CL is an update to one of the existing users of
ADDITIONAL_HEADERS to use this new argument instead, to serve as an
illustration of how this cleans up the CMake.

The big advantage of this new approach is that until now, there was no
way for the IDE projects to locate the header files that are in the
include tree.  In other words, if you are in, for example,
lib/DebugInfo/DWARF, the corresponding includes for this project will
be located under include/llvm/DebugInfo/DWARF.  Now, in the
CMakeLists.txt for lib/DebugInfo/DWARF, you can simply write:

  ADDITIONAL_HEADER_DIRS
  ../../include/llvm/DebugInfo/DWARF

as an argument to llvm_add_library(), and all header files will get
added to the IDE project.

Differential Revision: http://reviews.llvm.org/D7460
Reviewed By: Chris Bieneman

llvm-svn: 228670