[dwarfdump] Verify DW_AT_type is set and points to a compatible DIE.

This extends the verifier to catch three new errors:

  * Missing DW_AT_type attributes for DW_TAG_formal_parameter,
    DW_TAG_variable and DW_TAG_array_type.

  * Valid references for DW_AT_type pointing to a non-type tag.

Differential revision: https://reviews.llvm.org/D52223

llvm-svn: 342713