[Diagnostics] Reduce missing external macro diagnostic

This is a very large diagnostic, where the second half is mostly aimed
at macro authors rather than clients. Cut it down to the base
diagnostic.

Resolves rdar://113646544.

Author: bnbarham

include/swift/AST/DiagnosticsSema.def

@@ -7113,9 +7113,7 @@ ERROR(macro_undefined,PointsToFirstBadToken,
       "no macro named %0", (Identifier))
 ERROR(external_macro_not_found,none,
       "external macro implementation type '%0.%1' could not be found for "
-      "macro %2; the type must be public and provided by a macro target in a "
-      "Swift package, or via '-plugin-path' or '-load-plugin-library'",
-      (StringRef, StringRef, DeclName))
+      "macro %2", (StringRef, StringRef, DeclName))
 ERROR(macro_must_be_defined,none,
       "macro %0 requires a definition", (DeclName))
 ERROR(external_macro_outside_macro_definition,none,

test/Macros/macros_diagnostics.swift

@@ -136,7 +136,7 @@ func shadow(a: Int, b: Int, stringify: Int) {
 }
 
 func testMissing() {
-  #missingMacro1("hello") // expected-error{{external macro implementation type 'MissingModule.MissingType' could not be found for macro 'missingMacro1'; the type must be public and provided by a macro target in a Swift package, or via '-plugin-path' or '-load-plugin-library'}}
+  #missingMacro1("hello") // expected-error{{external macro implementation type 'MissingModule.MissingType' could not be found for macro 'missingMacro1'}}
 }
 
 @freestanding(expression) macro undefined() // expected-error{{macro 'undefined()' requires a definition}}