[Arm] Allow the use of arm_cde.h for C++

arm_cde.h includes the arm_mve_types.h header, which declares some C++
overloaded functions.

There is a superfluous `extern "C"` statement in arm_cde.h, which
encompasses these functions.  This means that if compiling for C++, the
overloaded functions are declared, but are declared without name
mangling.  Hence all the function names are the same and we have many
conflicting declarations.

Testing Done:
  Regression tested for arm-none-eabi.

gcc/ChangeLog:

2020-04-09  Matthew Malcomson  <matthew.malcomson@arm.com>

	* config/arm/arm_cde.h: Remove `extern "C"` when compiling for
	C++.

gcc/testsuite/ChangeLog:

2020-04-09  Matthew Malcomson  <matthew.malcomson@arm.com>

	* g++.target/arm/cde_mve.C: New test.

gcc/ChangeLog

+2020-04-09  Matthew Malcomson  <matthew.malcomson@arm.com>
+
+	* config/arm/arm_cde.h: Remove `extern "C"` when compiling for
+	C++.
+
 2020-04-09  Martin Jambor  <mjambor@suse.cz>
 	    Richard Biener  <rguenther@suse.de>
 

gcc/config/arm/arm_cde.h

@@ -27,10 +27,6 @@
 #ifndef _GCC_ARM_CDE_H
 #define _GCC_ARM_CDE_H 1
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 #include <stdint.h>
 
 #if defined (__ARM_FEATURE_CDE)
@@ -177,8 +173,4 @@ extern "C" {
 
 #endif
 
-#ifdef __cplusplus
-}
-#endif
-
 #endif

gcc/testsuite/ChangeLog

+2020-04-09  Matthew Malcomson  <matthew.malcomson@arm.com>
+
+	* g++.target/arm/cde_mve.C: New test.
+
 2020-04-09  Christophe Lyon  <christophe.lyon@linaro.org>
 
 	* gcc.target/arm/cmse/cmse-15.c: Use check-function-bodies.

gcc/testsuite/g++.target/arm/cde_mve.C

+/* { dg-do compile } */
+/* { dg-require-effective-target arm_v8_1m_main_cde_mve_fp_ok } */
+/* { dg-add-options arm_v8_1m_main_cde_mve_fp } */
+
+/* Ensure this compiles.  */
+#include "arm_cde.h"
+int foo ()
+{
+  return 1;
+}