Blender Git Loki

ASan/macOS: fix incomplete C/CXX compiler flags.

While testing for {rB40dcf686f04f}, compiler flags got mixed up and
non-working ASan configuration was committed.

Platform file, which is `include`d after the `CMAKE_C_FLAGS_DEBUG` etc.,
are set, overwrites those flags instead of appending to them. To fix this,
`PLATFORM_CFLAGS` is used to pass the `-fsanitize=*` flags to the C/C++
compiler.

Tested on fresh build using both Xcode and Ninja, with & without ccache.

Also silence a clang warning for multi-config generators:
the object size sanitizer has no effect at -O0, but is explicitly
enabled: -fsanitize=object-size [-Winvalid-command-line-argument]

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D8879