libstdc++: Suppress -Wstringop-overread warning in test

libstdc++: Suppress -Wstringop-overread warning in test

When compiled with -m32 -O2 -D_GLIBCXX_USE_CXX11_ABI=0 we get a warning
for 21_strings/basic_string/cons/char/1.cc:

bits/char_traits.h:409:56: warning: ‘void* __builtin_memcpy(void*, const void*, unsigned int)’ reading 1073741821 bytes from a region of size 19 [-Wstringop-overread]

The warning is legitimate, even if that line cannot be reached because
we throw std::length_error before getting there. Since the invalid
length is deliberate (and mentioned in a comment) just suppress the
warning, so that the test can verify we get the exception.

Also remove an unused typedef that produces another warning.

libstdc++-v3/ChangeLog:

	* testsuite/21_strings/basic_string/cons/char/1.cc: Use
	diagnostic pragma to suppress -Wstringop-overread error.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>