Issue diagnostics when returning FP values on x86_64 without SSE1/2

Avoid using report_fatal_error, because it will ask the user to file a
bug. If the user attempts to disable SSE on x86_64 and them use floating
point, that's a bug in their code, not a bug in the compiler.

This is just a start. There are other ways to crash the backend in this
configuration, but they should be updated to follow this pattern.

Differential Revision: https://reviews.llvm.org/D27522

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@302835 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGen/x86_64-mno-sse.c

@@ -0,0 +1,15 @@
+// RUN: %clang_cc1 -triple x86_64-linux -target-feature -sse -target-feature -sse2 -S -o /dev/null -verify %s
+// REQUIRES: x86-registered-target
+
+double f1(void) { // expected-error {{SSE register return with SSE disabled}}
+  return 1.4;
+}
+extern double g;
+void f2(void) { // expected-error {{SSE register return with SSE disabled}}
+  g = f1();
+}
+void take_double(double);
+void pass_double(void) {
+  // FIXME: Still asserts.
+  //take_double(1.5);
+}