RFA (tree-inline): PATCH for more C++14 constexpr support

[Jason Merrill]

This patch implements more support for C++14 constexpr: it allows 
arbitrary modification of variables in a constexpr function, but does 
not currently handle jumping -- multiple returns, loops, switches.

The approach I took for this was to just use the DECL_SAVED_TREE for a 
constexpr function as the basis for expansion rather than trying to 
massage it into a magic expression.  And now the values of local 
variables, including parameters, are kept in the values hash map that I 
introduced with the aggregate NSDMI patch.

But in the presence of recursive constexpr calls we can't use the same 
PARM_DECL as a key, so we need to remap it.  Thus I've added 
remap_fn_body to tree-inline.c to unshare the entire function body and 
remap the parms and result to avoid clashes.

This handles some more C++14 testcases and has no regressions on C++11 
constexpr testcases.  Support for jumps will follow soon.

Tested x86_64-pc-linux-gnu and powerpc64-unknown-linux-gnu.

Is the remap_fn_body function ok for trunk?
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