[ASTGen] Generate miscellaneous builtin pound

`#error`, `#warning`, and `#assert`.
Also #_hasSymbol statement condition

test/ASTGen/availability.swift

@@ -58,5 +58,7 @@ func testPoundIf() {
     // pass
   } else if #unavailable(macOS 80) {
     // pass
+  } else if #_hasSymbol(Int.self) { // expected-warning {{struct 'Int' is not a weakly linked declaration}}
+    // pass
   }
 }

test/ASTGen/diagnostics.swift

@@ -36,3 +36,18 @@ struct S {
 
 struct ExpansionRequirementTest<each T> {}
 extension ExpansionRequirementTest where repeat each T == Int {} // expected-error {{same-element requirements are not yet supported}}
+
+
+#warning("this is a warning") // expected-warning {{this is a warning}}
+
+func testDiagnosticInFunc() {
+  #error("this is an error") // expected-error {{this is an error}}
+}
+
+class TestDiagnosticInNominalTy {
+  #error("this is an error member") // expected-error {{this is an error member}}
+}
+
+#if FLAG_NOT_ENABLED
+  #error("error in inactive") // no diagnostis
+#endif

test/ASTGen/pound_assert.swift

@@ -0,0 +1,21 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend-dump-parse -enable-experimental-feature ParserASTGen \
+// RUN:   -enable-experimental-feature StaticAssert \
+// RUN:   | %sanitize-address > %t/astgen.ast
+// RUN: %target-swift-frontend-dump-parse \
+// RUN:   -enable-experimental-feature StaticAssert \
+// RUN:   | %sanitize-address > %t/cpp-parser.ast
+
+// RUN: %diff -u %t/astgen.ast %t/cpp-parser.ast
+
+// RUN: %target-swift-emit-sil -verify %s -enable-experimental-feature ParserASTGen \
+// RUN:   -enable-experimental-feature StaticAssert
+
+// REQUIRES: swift_swift_parser
+// REQUIRES: swift_feature_ParserASTGen
+// REQUIRES: swift_feature_StaticAssert
+
+func testPoundAssert() {
+#assert(1 == 1.0, "1 must be 1.0") // OK
+#assert(1 == 1.5, "1 is 1.5 ??") // expected-error {{1 is 1.5 ??}}
+}