Implement support for variable length arrays in C++. VLAs are limited

in several important ways:

  - VLAs of non-POD types are not permitted.
  - VLAs cannot be used in conjunction with C++ templates.

These restrictions are intended to keep VLAs out of the parts of the
C++ type system where they cause the most trouble. Fixes PR5678 and
<rdar://problem/8013618>.

llvm-svn: 104443