[ubsan] Detect invalid unsigned pointer index expression (clang)

Adding an unsigned offset to a base pointer has undefined behavior if
the result of the expression would precede the base. An example from
@regehr:

  int foo(char *p, unsigned offset) {
    return p + offset >= p; // This may be optimized to '1'.
  }

  foo(p, -1); // UB.

This patch extends the pointer overflow check in ubsan to detect invalid
unsigned pointer index expressions. It changes the instrumentation to
only permit non-negative offsets in pointer index expressions when all
of the GEP indices are unsigned.

Testing: check-llvm, check-clang run on a stage2, ubsan-instrumented
build.

Differential Revision: https://reviews.llvm.org/D33910

llvm-svn: 305216