gccrs: prealloc the initilizer vector

gccrs: prealloc the initilizer vector

There are two cases when initilizing an array, this is the
const context which means we need to build the array ctor,
which means using lots of memory, its super inefficient
because we are using a big wrapper over the GCC internals here
but preallocating the vectors here causes a:

  terminate called after throwing an instance of 'std::bad_alloc'

So this is a handy error condition to rely on for this senario.

Fixes Rust-GCC#3713
Fixes Rust-GCC#3727

gcc/rust/ChangeLog:

	* backend/rust-compile-expr.cc (CompileExpr::array_copied_expr): prealloc the vector

Signed-off-by: Philip Herron <herron.philip@googlemail.com>