Add verification for DW_AT_decl_file and DW_AT_call_file.

LTO builds have been creating invalid DWARF and one of the errors was a file index that was out of bounds. "llvm-dwarfdump --verify" will check all file indexes for line tables already, but there are no checks for the validity of file indexes in attributes.

The verification will verify if there is a DW_AT_decl_file/DW_AT_call_file that:
- there is a line table for the compile unit
- the file index is valid
- the encoding is appropriate

Tests are added that test all of the above conditions.

Differential Revision: https://reviews.llvm.org/D84817