string: Remove always_inline from basic_string::__init

This is effectively a partial revert of r278356, which started inlining
basic_string::__init.  Even if we want to help the compiler along with
an inlinehint, we shouldn't hamstring it by forcing it to inline all the
time.

Libc++ uses always_inline widely as a limit-the-ABI-hack, but since
__init is already on the dylib boundary, it makes no sense here and just
harms the debugging experience at -O0.

rdar://problem/31013102

llvm-svn: 299290