Rework the ConstStructBuilder code to emit missing initializer

elements with explicit zero values instead of with tail padding.
On an example like this:

struct foo { int a; int b; };

struct foo fooarray[] = {
    {1, 2},
    {4},
};

We now lay this out as:

@fooarray = global [2 x %struct.foo] [%struct.foo { i32 1, i32 2 }, %struct.foo { i32 4, i32 0 }]

instead of as:

@fooarray = global %0 <{ %struct.foo { i32 1, i32 2 }, %1 { i32 4, [4 x i8] zeroinitializer } }>

Preserving both the struct type of the second element, but also the array type of the entire thing.

llvm-svn: 101155