Fix Clang build issue with flexible array member and non-trivial dtor

Fix Clang build issue with flexible array member and non-trivial dtor

Commit d5cebea18e7a ("Make cached_reg_t own its data") added a
destructor to cached_reg_t.

That caused a build problem with Clang, which errors out like so:

 > CXX    python/py-unwind.o
 > gdb/python/py-unwind.c:126:16: error: flexible array member 'reg' of type 'cached_reg_t[]' with non-trivial destruction
 >   126 |   cached_reg_t reg[];
 >       |                ^

This is is not really a problem for our code, which allocates the
whole structure with xmalloc, and then initializes the array elements
with in-place new, and then takes care to call the destructor
manually.  Like, commit d5cebea18e7a did:

 @@ -928,7 +927,7 @@ pyuw_dealloc_cache (frame_info *this_frame, void *cache)
    cached_frame_info *cached_frame = (cached_frame_info *) cache;

    for (int i = 0; i < cached_frame->reg_count; i++)
 -    xfree (cached_frame->reg[i].data);
 +    cached_frame->reg[i].~cached_reg_t ();

Maybe we should get rid of the flexible array member and use a bog
standard std::vector.  I doubt this would cause any visible
performance issue.

Meanwhile, to unbreak the build, this commit switches from C99-style
flexible array member to 0-length array.  It behaves the same, and
Clang doesn't complain.  I got the idea from here:

  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70932#c11

GCC 9, our oldest support version, already supported this:

  https://gcc.gnu.org/onlinedocs/gcc-9.1.0/gcc/Zero-Length.html

but the extension is actually much older than that.  Note that
C99-style flexible array members are not standard C++ either.

Change-Id: I37dda18f367e238a41d610619935b2a0f2acacce