[DebugInfo] Add stringLocationExp field to DIStringType

DIStringType is used to encode the debug info of a character object
in Fortran. A Fortran deferred-length character object is typically
implemented as a pair of the following two pieces of info: An address
of the raw storage of the characters, and the length of the object.
The stringLocationExp field contains the DIExpression to get to the
raw storage.

This patch also enables the emission of DW_AT_data_location attribute
in a DW_TAG_string_type debug info entry based on stringLocationExp
in DIStringType.

A test is also added to ensure that the bitcode reader is backward
compatible with the old DIStringType format.

Differential Revision: https://reviews.llvm.org/D117586