[XCOFF][DWARF] XCOFF64 should be able to select the dwarf format in intergrated-as mode.

Summary: DWARF32 is not supported for XCOFF64 under non-integrated-as mode on AIX, because system assembler will fill the debug section lengths according to DWARF64 format. While in intergrated-as mode, XCOFF64 should be able to select the DWARF format.

Reviewed By: shchenz

Differential Revision: https://reviews.llvm.org/D150181