[cxx-interop] Use the locations imported from C++

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.

lib/AST/Decl.cpp

@@ -1011,6 +1011,7 @@ static_assert(sizeof(checkSourceLocType(&ID##Decl::getLoc)) == 2, \
     return getLocFromSource();
   switch(File->getKind()) {
   case FileUnitKind::Source:
+  case FileUnitKind::ClangModule:
     return getLocFromSource();
   case FileUnitKind::SerializedAST: {
     if (!SerializedOK)
@@ -1019,7 +1020,6 @@ static_assert(sizeof(checkSourceLocType(&ID##Decl::getLoc)) == 2, \
   }
   case FileUnitKind::Builtin:
   case FileUnitKind::Synthesized:
-  case FileUnitKind::ClangModule:
   case FileUnitKind::DWARFModule:
     return SourceLoc();
   }