[LLVM] [cmake] Ensure proper -I along with -include on SunOS, #29013

[mgorny]

Summary

As described on the bug, the '-include' flag added to C++ flags on SunOS gets propagated to subprojects such as libc++ while they do not have matching include directories enabled. This simple patch adds an -I flag along with the -include to ensure that the compiler is able to find the requested include.

Bug: https://llvm.org/bugs/show_bug.cgi?id=29013

[mgorny]

@EricWF, ping.

[EricWF]

@mgorny: I would suggest adding some reviews that frequently review LLVM CMake changes since this change will affect more than just libc++.

CMakeLists.txt

 if(${CMAKE_SYSTEM_NAME} MATCHES "(FreeBSD|DragonFly)")
   # On FreeBSD, /usr/local/* is not used by default. In order to build LLVM
   # with libxml2, iconv.h, etc., we must add /usr/local paths.
   include_directories("/usr/local/include")
   link_directories("/usr/local/lib")
 endif(${CMAKE_SYSTEM_NAME} MATCHES "(FreeBSD|DragonFly)")
 
 if( ${CMAKE_SYSTEM_NAME} MATCHES SunOS )
-   SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -include llvm/Support/Solaris.h")
+   SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -I${CMAKE_SOURCE_DIR}/include -include llvm/Support/Solaris.h")
 endif( ${CMAKE_SYSTEM_NAME} MATCHES SunOS )
 
 # Make sure we don't get -rdynamic in every binary. For those that need it,
 # use export_executable_symbols(target).
 set(CMAKE_SHARED_LIBRARY_LINK_CXX_FLAGS "")
 
 set(LLVM_PROFDATA_FILE "" CACHE FILEPATH
   "Profiling data file to use when compiling in order to improve runtime performance.")