[wasm] add @_expose(wasm) attribute support

This attribute instructs the compiler that this function declaration
should be "export"ed from this .wasm module. It's equivalent of Clang's
`__attribute__((export_name("name")))`

include/swift/SIL/SILFunction.h

@@ -299,6 +299,10 @@ private:
   /// Name of a section if @_section attribute was used, otherwise empty.
   StringRef Section;
 
+  /// Name of a Wasm export if @_expose(wasm) attribute was used, otherwise
+  /// empty.
+  StringRef WasmExportName;
+
   /// Has value if there's a profile for this function
   /// Contains Function Entry Count
   ProfileCounter EntryCount;
@@ -1264,6 +1268,10 @@ public:
   StringRef section() const { return Section; }
   void setSection(StringRef value) { Section = value; }
 
+  /// Return Wasm export name if @_expose(wasm) was used, otherwise empty
+  StringRef wasmExportName() const { return WasmExportName; }
+  void setWasmExportName(StringRef value) { WasmExportName = 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.