Retry: [ubsan] Detect UB loads from bitfields

It's possible to load out-of-range values from bitfields backed by a
boolean or an enum. Check for UB loads from bitfields.

This is the motivating example:

  struct S {
    BOOL b : 1; // Signed ObjC BOOL.
  };

  S s;
  s.b = 1; // This is actually stored as -1.
  if (s.b == 1) // Evaluates to false, -1 != 1.
    ...

Changes since the original commit:

- Single-bit bools are a special case (see CGF::EmitFromMemory), and we
  can't avoid dealing with them when loading from a bitfield. Don't try to
  insert a check in this case.

Differential Revision: https://reviews.llvm.org/D30423

llvm-svn: 297389

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