[DWARFv5] Fuss with asm syntax for conveying MD5 checksum.

[DWARFv5] Fuss with asm syntax for conveying MD5 checksum.

Previously the MD5 option of the .file directive provided the checksum
as a quoted hex string; now it's a normal hex number with 0x prefix,
same as the .octa directive accepts.

Differential Revision: https://reviews.llvm.org/D45459

llvm-svn: 329820