Haven't yet found a nice way to handle TargetData verification in the

AsmParser. This patch adds validation for target data layout strings upon
construction of TargetData objects. An attempt to construct a TargetData object
from a malformed string will trigger an assertion.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142605 91177308-0d34-0410-b5e6-96231b3b80d8