Improve Assert output

This kind of output is usually not helpful:

TRAP: BadArgument("((context) != ((void *)0) && (((((const
Node*)((context)))->type) == T_AllocSetContext) || ((((const
Node*)((context)))->type) == T_SlabContext) || ((((const
Node*)((context)))->type) == T_GenerationContext)))", File:
"../../../../src/include/utils/memutils.h", Line: 129)

What we probably want is something like:

TRAP: BadArgument("MemoryContextIsValid(context)", File:
"../../../../src/include/utils/memutils.h", Line: 129)

The problem is that the way the Assert macros are written they
macro-expand the arguments before stringifying them.  The attached patch
fixes that.  This requires both replacing CppAsString by plain "#" and
not redirecting Assert() to Trap().

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
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