Use __builtin_FILE instead of __FILE__ in assert in C++.

Likewise use __builtin_LINE instead of __LINE__.

When building C++, inline functions are required to have the exact same
sequence of tokens in every translation unit. But __FILE__ token, when
used in a header file, does not necessarily expand to the exact same
string literal, and that may cause compilation failure when C++ modules
are being used. (It would also cause unpredictable output on assertion
failure at runtime, but this rarely matters in practice.)

For example, given the following sources:

  // a.h
  #include <assert.h>
  inline void fn () { assert (0); }

  // a.cc
  #include "a.h"

  // b.cc
  #include "foo/../a.h"

preprocessing a.cc will yield a call to __assert_fail("0", "a.h", ...)
but b.cc will yield __assert_fail("0", "foo/../a.h", ...)

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