Change session log filename pattern: test before arch

This is a very minor change.

Just reshuffle the elements of the log filename so that the test name
comes before the arch/compiler

Before:
Failure-x86_64-clang-TestCModules.CModulesTestCase.test_expr_with_dwarf.log

After:
Failure-TestCModules.CModulesTestCase.test_expr_with_dwarf-x86_64-clang.log

This way, results are grouped by test name, not by arch. I think this
is much more useful.

Differential Revision: http://reviews.llvm.org/D9648

llvm-svn: 237076