Fix pre-6.0 build

Author: kabiroberai

Sources/SwiftCompilerPlugin/CompilerPlugin.swift

@@ -313,14 +313,13 @@ private var readabilityHandler: () -> Void = {
   """)
 }
 
-#if compiler(>=6.0)
-
-@_expose(wasm, "swift_wasm_macro_pump")
-@_cdecl("swift_wasm_macro_pump")
+// We can use @_expose(wasm, ...) with compiler >= 6.0
+// but it causes build errors with older compilers.
+// Instead, we export from wasm_support.c and trampoline
+// to this method.
+@_cdecl("_swift_wasm_macro_pump")
 func wasmPump() {
   readabilityHandler()
 }
 
 #endif
-
-#endif

Sources/_SwiftSyntaxCShims/wasm_support.c

@@ -22,4 +22,11 @@
 // use __attribute__((section)) for it
 __asm__("\t.section .custom_section.swift_wasm_macro_abi,\"\",@\n\t.4byte " STR(SWIFT_WASM_ABI_VERSION_RAW) "\n");
 
+void _swift_wasm_macro_pump(void);
+
+__attribute__((export_name("swift_wasm_macro_pump")))
+void swift_wasm_macro_pump(void) {
+    _swift_wasm_macro_pump();
+}
+
 #endif