when compiling in a GNU mode (e.g. gnu99) treat VLAs with a size that can be folded to a constant

as constant size arrays.  This has slightly different semantics in some insane cases, but allows
us to accept some constructs that GCC does.  Continue to be pedantic in -std=c99 and other
modes.  This addressed rdar://8733881 - error "variable-sized object may not be initialized"; g++ accepts same code


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@132983 91177308-0d34-0410-b5e6-96231b3b80d8