[flang] Take the debug record's location from the declaration. (#229484)

`DeclareOpConversion` gives the `DbgDeclareOp` it creates the location
of the memref (the alloca) instead of the location of the `XDeclareOp`.
This is problematic in 2 ways.

1. The `XDeclareOp` better represents where the variable is declared.
The memref is where the variable's memory happens to be allocated, and
for a dummy argument it is a block argument, so the record ends up on
the procedure statement. For a subroutine whose dummies `n` and `a` are
declared on lines 2 and 3, their `#dbg_declare` records are both at line
1 today, and at lines 2 and 3 with this change.

2. It requires the alloca to have a valid location. If it does not, the
`DbgDeclareOp` gets none either, and a debug record without a location
is invalid, so it is dropped during the MLIR to LLVM IR translation and
the variable disappears from the debug info altogether.

This PR makes `DeclareOpConversion` use the location of the
`XDeclareOp`. A similar change is done for `DeclareValueOpConversion`.

Assisted By: Cursor

Co-authored-by: Cursor <cursoragent@cursor.com>