cmake: Add Include What You Use support

Include What You Use (IWYU) is a tool from Google dedicated to
sanitizing header inclusions. It detects overly-broad inclusion of
headers and attempts to reduce the scope in a way that you only "include
what you use".

Like with CPPCheck and Clang-Tidy, this is run alongside the compiler.

Change-Id: Idaa495cb003a7ccbaaea7d8cdea3bdbbce4594af
Signed-off-by: Chris Kay <chris.kay@arm.com>
Signed-off-by: Joel Goddard <joel.goddard@arm.com>
Signed-off-by: Girish Pathak <girish.pathak@arm.com>