[Ada] Create object file for ignored ghost compilation unit

[Pierre-Marie de Rodat]

If the compilation unit we're compiling is an ignored ghost, we need to
ensure that an object file is generated. This happens automatically for
the GCC backend since it always generates the .o file, but for GNAT
LLVM, the backend isn't called. So we need to be sure that we explicitly
generate the file.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* gnat1drv.adb (Gnat1drv): Force generation of object file if
	compilation unit is an ignored ghost.
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