Enable msan unconditionally on Linux.

Enable msan unconditionally on Linux.

Memory sanitizer compatibility are already done in
MemorySanitizer::doInitialization. It verifies whether the necessary offsets
exist and bails out if not. For this reason it is no good to duplicate two
checks in two projects. This patch removes clang check and postpones msan
compatibility validation till MemorySanitizer::doInitialization.

Another reason for this patch is to allow using msan with any CPU (given
compatible runtime) and custom mapping provided via the arguments added by
https://reviews.llvm.org/D44926.

Patch by vit9696.

Differential Revision: https://reviews.llvm.org/D44927

llvm-svn: 329241