[DebugInfo] Add handling of stringLengthExp operand of DIStringType.

This patch makes DWARF writer emit DW_AT_string_length using
the stringLengthExp operand of DIStringType.

This is part of the effort to add debug info support for
Fortran deferred length strings.

Also updated the tests to exercise the change.

Differential Revision: https://reviews.llvm.org/D92412