fix PR17635: false positive with packed structures
LLVM optimizers may widen accesses to packed structures that overflow the structure itself, but should be in bounds up to the alignment of the object
Teach Clang to recognize and ignore GCC's -fdiagnostics-show-location
flag. We should probably wire at least some variants of this up to our
actual diagnostics engine, but I'm leaving that for someone else. This
fixes the builds of packages which hard code something here, at least
including libstdc++ itself.