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.
The diagnostics engine has some code to pretty-print a declaration when
there is no source location for that declaration. The declaration is
pretty-printed into a source buffer, and a source location into that
buffer is synthesizes. This applies to synthesized declarations as well
as those imported from Swift modules (without source code) or from Clang.
Reimplement this pretty-printing for declarations as a request. In
doing so, change the manner in which we do the printing: the
diagnostics engine printed the entire enclosing type into a buffer
whose name was the module + that type. This meant that the buffer was
shared by every member of that type, but also meant that we would end
up deserializing a lot of declarations just for printing and
potentially doing a lot more work for these diagnostics.
This modifies the ClangImporter to introduce an opaque placeholder
representation for forward declared Objective-C interfaces and
protocols when imported into Swift.
In the compiler, the new functionality is hidden behind a frontend
flag -enable-import-objc-forward-declarations, and is on by default
for language mode >6.
The feature is disabled entirely in LLDB expression evaluation / Swift
REPL, regardless of language version.
Replace the existing `-enable-experimental-clang-importer-diagnostics`
flag with an opt-out version entitled `-disable-experimentalc-clang-importer-diagnostics`.
Enable the beviour previously hidden behind the old flag by default.
