This removes the -cxx-abi flag and makes the ABI depend just on the triple. It doesn't make sense to compile with "-triple i686-pc-win32 -cxx-abi itanium", so there's no need for this extra knob.
To be able to run a test with a specific ABI without constraining it to a specific triple, new substitutions are added to lit: %itanium_abi_triple and %ms_abi_triple can be used to get the current target triple adjusted to the desired ABI. For example, if the test suite is running with i686-pc-win32 target, %itanium_abi_triple will expand to i686-pc-mingw32.
Apologies for the large number of test updates. It's mostly mechanical, though :)