[cfe-dev] Implementation of mode attribute

[Eli Friedman]

Per subject, implementation of the gcc mode attribute (e.g. int x
__attribute__((mode(HI)));).  Most significant usage is in the glibc
headers to implement intN_t and friends (PR2204).

The implementation is pretty straightforward: it modifies the type of
the declaration to correspond to the type corresponding to the
specified mode.  Currently, the mappings are hardcoded, and only
completely correct for X86.  We should probably add some way to get
integers/floats of specific widths from the ASTContext.

There's also a way to use the mode attribute to specify vectors, but
per gcc it's deprecated, and there aren't any headers on my system
using it.  If we do end up needing it, it's should be easy to add.

One bug that's kind of out of the scope of this patch: we're currently
processing attributes in the wrong order.  For example, take the
following declaration:
float x __attribute((mode(DF),vector_size(16)));
This should declare a variable with the LLVM type <2 x double> (a
vector of size 16 with DFMode elements), but that doesn't work with
this patch because we process the vector_size attribute first.  I
haven't actually seen any code which does this, though, so I won't
worry about it for the moment.

-Eli
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