A recent PR (#77204) started to import C++ source locations into Swift.
This PR flips a switch so these locations are actually used more widely.
Now some of the diagnostic locations are changed, but they generally
improved the quality of the diagnostics, pointing out conformances
imported from Obj-C code right when they are declared.
In C, a function can be declared at local scope:
void aFunctionInBase(void) {
void theFunctionInQuestion(int);
}
and then again in a different header at top-level scope:
void theFunctionInQuestion(int);
If the first one appears first, it becomes what Clang considers the
"canonical" declaration, which (up until now) Swift has been using to
decide what module to import a function into. (Since a C function can
be redeclared arbitrarily many times, we have to pick one.) This is
important for diagnostics and anything else that might ask "where did
this Swift declaration come from". Instead of the very first
redeclaration, use the first non-local one to determine the "home"
module.
(The standard library wants a guarantee that forward declarations they
put in SwiftShims won't interfere with declarations found elsewhere. I
don't think Clang can /ever/ provide that, so if there's ever a
mismatch between the standard library's forward declarations and the
"real" declarations the standard library's might win out at the LLVM
level---say, in terms of attributes. But this at least removes a place
where that could be visible to users even when it isn't otherwise a
problem.)
