[llvm] support graceful failure of DataLayout parsing

Existing implementation always aborts on syntax errors in a DataLayout
description. While this is meaningful for consuming textual IR modules, it is
inconvenient for users that may need fine-grained control over the layout from,
e.g., command-line options. Propagate errors through the parsing functions and
only abort in the top-level parsing function instead.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D85650