NFC: Pre-commit test: -Wpointer-sign with plain char to [un]signed char

Add tests with bad message text for `-Wpointer-sign` and run them with
both signed and unsigned versions of plain `char`.

clang/test/Sema/incompatible-sign.c

@@ -1,5 +1,23 @@
 // RUN: %clang_cc1 %s -verify -fsyntax-only
+// RUN: %clang_cc1 %s -verify -fsyntax-only -fno-signed-char
 
 int a(int* x); // expected-note{{passing argument to parameter 'x' here}}
 int b(unsigned* y) { return a(y); } // expected-warning {{passing 'unsigned int *' to parameter of type 'int *' converts between pointers to integer types with different sign}}
 
+signed char *plainCharToSignedChar(signed char *arg) { // expected-note{{passing argument to parameter}}
+  extern char c;
+  signed char *p = &c; // expected-warning {{converts between pointers to integer types with different sign}}
+  struct { signed char *p; } s = { &c }; // expected-warning {{converts between pointers to integer types with different sign}}
+  p = &c; // expected-warning {{converts between pointers to integer types with different sign}}
+  plainCharToSignedChar(&c); // expected-warning {{converts between pointers to integer types with different sign}}
+  return &c; // expected-warning {{converts between pointers to integer types with different sign}}
+}
+
+char *unsignedCharToPlainChar(char *arg) { // expected-note{{passing argument to parameter}}
+  extern unsigned char uc[];
+  char *p = uc; // expected-warning {{converts between pointers to integer types with different sign}}
+  (void) (char *[]){ [42] = uc }; // expected-warning {{converts between pointers to integer types with different sign}}
+  p = uc; // expected-warning {{converts between pointers to integer types with different sign}}
+  unsignedCharToPlainChar(uc); // expected-warning {{converts between pointers to integer types with different sign}}
+  return uc; // expected-warning {{converts between pointers to integer types with different sign}}
+}

clang/test/Sema/incompatible-sign.cpp

@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 %s -verify -fsyntax-only
+// RUN: %clang_cc1 %s -verify -fsyntax-only -fno-signed-char
+
+void plainToSigned() {
+  extern char c;
+  signed char *p;
+  p = &c; // expected-error {{converts between pointers to integer types with different sign}}
+}
+
+void unsignedToPlain() {
+  extern unsigned char uc;
+  char *p;
+  p = &uc; // expected-error {{converts between pointers to integer types with different sign}}
+}