ARM: add VLA extension for WoA Itanium ABI

The armv7-windows-itanium environment is nearly identical to the MSVC ABI. It
has a few divergences, mostly revolving around the use of the Itanium ABI for
C++. VLA support is one of the extensions that are amongst the set of the
extensions.

This adds support for proper VLA emission for this environment. This is
somewhat similar to the handling for __chkstk emission on X86 and the large
stack frame emission for ARM. The invocation style for chkstk is still
controlled via the -mcmodel flag to clang.

Make an explicit note that this is an extension.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210489 91177308-0d34-0410-b5e6-96231b3b80d8