move sai fake to sai 1.12.0

Summary:
X-link: facebook/fboss#147

As titled, move sai fake sai to 1.12.0. BCM and TAJO P4 WB SDK already moved to 1.12.0 and its good to move fake also to the same version.

This diff addresses:

- Moving SAI version for fake from sai version 1.7 to 1.12
- Update sandcastle job to build the fake sai 1.12
- saiobject.h includes experimental header files as part of SAI 1.12 and hence the include path has to contain both sai/include and sai/experimental. Currently, vendors leverage the experimental header files differently (by placing include and experimental in the same include dir) than SAI spec but eventually we will have vendors also converge to the same model. Changing it now will break BCM builds.
- Ignore including Saiversion.h in OSS since 1.12 contains saiversion.h already and defines SAI_VERSION and SAI_API_VERSION macros.

NOTE that this should not alter BCM OSS builds.

Reviewed By: shri-khare

Differential Revision: D47779384

fbshipit-source-id: ba10a35dd95306c908c5cd3f76a1485461b011d7

build/fbcode_builder/manifests/libsai

@@ -2,12 +2,13 @@
 name = libsai
 
 [download]
-url = https://github.com/opencomputeproject/SAI/archive/v1.7.1.tar.gz
-sha256 = e18eb1a2a6e5dd286d97e13569d8b78cc1f8229030beed0db4775b9a50ab6a83
+url = https://github.com/opencomputeproject/SAI/archive/v1.12.0.tar.gz
+sha256 = 1e7f43599baf1dcca122bbbb2baaeb9b20e5632d2ca6aaa61a568d1d58afaa97
 
 [build]
 builder = nop
-subdir = SAI-1.7.1
+subdir = SAI-1.12.0
 
 [install.files]
 inc = include
+experimental = experimental