[LLDB][MIPS] Fix TestConstVariables.py

Patch by Nitesh Jain.

Summary: There is no debug information generated for variable index with –O3 optimization flag. The DW_AT_location tag in DWARF debug_info section is empty.

Reviewers: ovyalov, clayborg
Subscribers: lldb-commits, mohit.bhakkad, sagar, bhushan, jaydeep
Differential: http://reviews.llvm.org/D15224
llvm-svn: 254850