Rewrite of isIntegerConstantExpr to be centered around Evaluate. This

is a rather big change, but I think this is the direction we want to go; 
the code is significantly shorter now, and it doesn't duplicate Evaluate 
code.  There shouldn't be any visible changes as far as I know.

There has been some movement towards putting ICE handling into 
Evaluate (for example, VerifyIntegerConstantExpression uses Evaluate 
instead of isICE).  This patch is sort of the opposite of the approach, 
making ICE handling work without Evaluate being aware of it. I think 
this approach is better because it separates the code that does the 
constant evaluation from code that's calculating a rather 
arbitrary predicate.

The one thing I don't really like about this patch is that 
the handling of commas in C99 complicates it signficantly. (Seriously, 
what was the standards committee thinking when they wrote that 
part?) I think I've come up with a decent approach, but it doesn't feel
ideal.  I might add some way to check for evaluated commas from Evaluate 
in a subsequent patch; that said, it might not be worth bothering.

llvm-svn: 65524