Variable auto-init: don't initialize aggregate padding of all aggregates

Summary:
C guarantees that brace-init with fewer initializers than members in the
aggregate will initialize the rest of the aggregate as-if it were static
initialization. In turn static initialization guarantees that padding is
initialized to zero bits.

Quoth the Standard:

C17 6.7.9 Initialization ❡21

If there are fewer initializers in a brace-enclosed list than there are elements
or members of an aggregate, or fewer characters in a string literal used to
initialize an array of known size than there are elements in the array, the
remainder of the aggregate shall be initialized implicitly the same as objects
that have static storage duration.

C17 6.7.9 Initialization ❡10

If an object that has automatic storage duration is not initialized explicitly,
its value is indeterminate. If an object that has static or thread storage
duration is not initialized explicitly, then:

 * if it has pointer type, it is initialized to a null pointer;
 * if it has arithmetic type, it is initialized to (positive or unsigned) zero;
 * if it is an aggregate, every member is initialized (recursively) according to
   these rules, and any padding is initialized to zero bits;
 * if it is a union, the first named member is initialized (recursively)
   according to these rules, and any padding is initialized to zero bits;

<rdar://problem/50188861>

Reviewers: glider, pcc, kcc, rjmccall, erik.pilkington

Subscribers: jkorous, dexonsmith, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61280

llvm-svn: 359628