Rename Sema::isConstantArrayType -> VerifyConstantArrayType. Make sure to

check canonical types in a few places.  Tighten up VerifyConstantArrayType
to diagnose more errors, now that we can evaluate i-c-e's.  Add some fixmes
about poor diagnostics.

We now correctly typecheck this example:

void s(void) {
  typedef int a[(int) +1.0];
  static a b;    // invalid, static VLA

  static int c[(int) +1.0];  // invalid, static VLA
}

void t(void) {
  typedef int a[(int)1.0];
  static a b;    // valid, constant size
}

void u() {
  static int X[-1];
  static int Y[0];
}

producing:

static-vla.c:3:12: error: variable length array declared outside of any function
  static a b;    // invalid, static VLA
           ^
static-vla.c:5:14: error: variable length array declared outside of any function
  static int c[(int) +1.0];  // invalid, static VLA
             ^ ~~~~~~~~~~
static-vla.c:15:14: error: array size is negative
  static int X[-1];
             ^ ~~
static-vla.c:16:14: warning: zero size arrays are an extension
  static int Y[0];
             ^ ~

llvm-svn: 39587