[Ada] Crash on exported build-in-place function

[Ada] Crash on exported build-in-place function

This patch fixes a bug where if a function is build-in-place, and is
exported, and contains an extended_return_statement whose object is
initialized with another build-in-place function call, then the compiler
will crash.

2019-07-05  Bob Duff  <duff@adacore.com>

gcc/ada/

	* exp_ch6.adb (Is_Build_In_Place_Function): Narrow the check for
	Has_Foreign_Convention to the imported case only.  If a
	build-in-place function is exported, and called from Ada code,
	build-in-place protocols should be used.

gcc/testsuite/

	* gnat.dg/bip_export.adb, gnat.dg/bip_export.ads: New testcase.

From-SVN: r273113