[Matrix] Add test showing unintended implicit sign conversion warning.

Author: fhahn

clang/test/SemaCXX/matrix-index-operator-sign-conversion.cpp

@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 -triple arm64-apple-macosx -std=c++11 -fenable-matrix -fsyntax-only -verify -Wsign-conversion %s
+
+template <typename T, int R, int C> using m __attribute__((__matrix_type__(R,C))) = T;
+
+// FIXME: should not warn here.
+double index1(m<double,3,1> X, int      i) { return X[i][0]; }
+// expected-warning@-1 {{implicit conversion changes signedness: 'int' to 'unsigned long'}}
+
+double index2(m<double,3,1> X, unsigned i) { return X[i][0]; }
+
+double index3(m<double,3,1> X, char     i) { return X[i][0]; }
+// expected-warning@-1 {{implicit conversion changes signedness: 'char' to 'unsigned long'}}
+
+double index4(m<double,3,1> X, int      i) { return X[0][i]; }
+// expected-warning@-1 {{implicit conversion changes signedness: 'int' to 'unsigned long'}}
+
+double index5(m<double,3,1> X, unsigned i) { return X[0][i]; }
+
+double index6(m<double,3,1> X, char     i) { return X[0][i]; }
+// expected-warning@-1 {{implicit conversion changes signedness: 'char' to 'unsigned long'}}