My apologies for the large patch. With the exception of ConstString.h itself it was entirely produced by sed.
I technically only need one of these changes to fix a taking-reference-of-local-object bug that Jason discovered on the swift-lldb branch, but while I'm at it I thought I might as well do all at once.
ConstString has exactly one const char * data member, so passing a ConstString by reference is not any more efficient than copying it by value. In both cases a single pointer is passed. But passing it by value makes it harder to accidentally return the address of a local object.
(This fixes rdar://problem/48640859 for the Apple folks)