string-buffer: Enable resource leak warnings from clang.
* m4/gnulib-common.m4 (gl_COMMON_BODY): Define the macros
_GL_ATTRIBUTE_CAPABILITY_TYPE, _GL_ATTRIBUTE_ACQUIRE_CAPABILITY,
_GL_ATTRIBUTE_RELEASE_CAPABILITY.
* lib/string-buffer.h (sb_heap_allocated_pointer_t): New type.
(struct string_buffer): Use it.
(sb_init): Mark with _GL_ATTRIBUTE_ACQUIRE_CAPABILITY.
(sb_free, sb_dupfree): Mark with _GL_ATTRIBUTE_RELEASE_CAPABILITY.
* lib/string-buffer.c: Silence the -Wthread-safety warnings in this
compilation unit.