Add a simulate_builin_function_decl langhook

Add a simulate_builin_function_decl langhook

Although it's possible to define the SVE intrinsics in a normal header
file, it's much more convenient to define them directly in the compiler.
This also speeds up compilation and gives better error messages.

The idea is therefore for arm_sve.h (the main intrinsics header file)
to have the pragma:

    #pragma GCC aarch64 "arm_sve.h"

telling GCC to define (almost) everything arm_sve.h needs to define.
The target then needs a way of injecting new built-in function
declarations during compilation.

The main hook for defining built-in functions is add_builtin_function.
This is designed for use at start-up, and so has various features that
are correct in that context but not for the pragma above:

  (1) the location is always BUILTINS_LOCATION, whereas for arm_sve.h
      it ought to be the location of the pragma.

  (2) the function is only immediately visible if it's in the implementation
      namespace, whereas the pragma is deliberately injecting functions
      into the general namespace.

  (3) there's no attempt to emulate a normal function declaration in
      C or C++, whereas functions declared by the pragma should be
      checked in the same way as an open-coded declaration would be.
      E.g. we should get an error if there was a previous incompatible
      declaration.

  (4) in C++, the function is treated as extern "C" and so can't be
      overloaded, whereas SVE intrinsics do use function overloading.

This patch therefore adds a hook that targets can use to inject
the equivalent of a source-level function declaration, but bound
to a BUILT_IN_MD function.

The main SVE intrinsic patch has tests to make sure that we report an
error for conflicting definitions that appear either before or after
including arm_sve.h.

2019-10-29  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* langhooks.h (lang_hooks::simulate_builtin_function_decl): New hook.
	(simulate_builtin_function_decl): Declare.
	* langhooks-def.h (LANG_HOOKS_SIMULATE_BUILTIN_FUNCTION_DECL): Define.
	(LANG_HOOKS_INITIALIZER): Include it.
	* langhooks.c (add_builtin_function_common): Rename to...
	(build_builtin_function): ...this.  Add a location parameter and use
	it instead of BUILTINS_LOCATION.  Remove the hook parameter and return
	the decl instead.
	(add_builtin_function): Update accordingly, passing the returned
	decl to the lang hook.
	(add_builtin_function_ext_scope): Likewise
	(simulate_builtin_function_decl): New function.

gcc/c/
	* c-tree.h (c_simulate_builtin_function_decl): Declare.
	* c-decl.c (c_simulate_builtin_function_decl): New function.
	* c-objc-common.h (LANG_HOOKS_SIMULATE_BUILTIN_FUNCTION_DECL): Define
	to the above.

gcc/cp/
	* cp-tree.h (cxx_simulate_builtin_function_decl): Declare.
	* decl.c (cxx_simulate_builtin_function_decl): New function.
	* cp-objcp-common.h (LANG_HOOKS_SIMULATE_BUILTIN_FUNCTION_DECL): Define
	to the above.

From-SVN: r277554