Add `DW_AT_pure` attribute for `pure` functions

[JohanEngelen]

We can use Dwarf's pure function attribute to mark pure functions as such in debuggers.

See https://reviews.llvm.org/D54043

(for more context: https://lists.llvm.org/pipermail/llvm-dev/2018-November/127419.html)

1. Elemental, Pure, and Recursive Procedures

DWARF 4 defines attributes for these Fortran procedure specifiers:
DW_AT_elemental, DW_AT_pure, DW_AT_recursive, resp. LLVM has a way of
informing the DWARF generator of simple boolean attributes in the metadata
via the flags parameter. We have added these new values to the existing
collection of flags.

!61 = !DISubprogram(…, flags: DIFlagPure)