Add @_used and @_section attributes for global variables and top-level functions (#65901)

* Add @_used and @_section attributes for global variables and top-level functions

This adds:
- @_used attribute that flags as a global variable or a top-level function as
  "do not dead-strip" via llvm.used, roughly the equivalent of
  __attribute__((used)) in C/C++.
- @_section("...") attribute that places a global variable or a top-level
  function into a section with that name, roughly the equivalent of
  __attribute__((section("..."))) in C/C++.

include/swift/SIL/SILFunction.h

@@ -291,6 +291,9 @@ private:
   /// The function's remaining set of specialize attributes.
   std::vector<SILSpecializeAttr*> SpecializeAttrSet;
 
+  /// Name of a section if @_section attribute was used, otherwise empty.
+  StringRef Section;
+
   /// Has value if there's a profile for this function
   /// Contains Function Entry Count
   ProfileCounter EntryCount;
@@ -346,6 +349,9 @@ private:
   /// would indicate.
   unsigned HasCReferences : 1;
 
+  /// Whether attribute @_used was present
+  unsigned MarkedAsUsed : 1;
+
   /// Whether cross-module references to this function should always use weak
   /// linking.
   unsigned IsAlwaysWeakImported : 1;
@@ -1234,6 +1240,14 @@ public:
     return V && V->getAttrs().hasAttribute<AlwaysEmitIntoClientAttr>();
   }
 
+  /// Return whether this function has attribute @_used on it
+  bool markedAsUsed() const { return MarkedAsUsed; }
+  void setMarkedAsUsed(bool value) { MarkedAsUsed = value; }
+
+  /// Return custom section name if @_section was used, otherwise empty
+  StringRef section() const { return Section; }
+  void setSection(StringRef value) { Section = value; }
+
   /// Returns true if this function belongs to a declaration that returns
   /// an opaque result type with one or more availability conditions that are
   /// allowed to produce a different underlying type at runtime.