Modified the expression parser to only try to
write to registers if they were modified in the
expression.  This eliminates spurious errors if
the register can't be written to but the
expression didn't write to it anyway.

Also improved error handling for the materializer
to make "couldn't materialize struct" errors more
informative.

<rdar://problem/14322579>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@186228 91177308-0d34-0410-b5e6-96231b3b80d8