[asan] Add CMake hook to override shadow scale in compiler_rt

Allow user to override shadow scale in compiler_rt by passing
-DCOMPILER_RT_ASAN_SHADOW_SCALE=n to CMake.  Propagate the override
shadow scale value via a compiler define to compiler-rt and asan
tests.  Tests will use the define to partially disable unsupported
tests.  Set "-mllvm -asan-mapping-scale=<n>" for compiler_rt tests.

Differential Revision: https://reviews.llvm.org/D39469

llvm-svn: 318038