report compiling error:

Error 13 error C2660: ‘llvm::APFloat::convert’ : function does not take 3 arguments \lib\AST\APValue.cpp 70
with conflicted definition and usage:
in the APFloat.h 219
opStatus convert(const fltSemantics &, roundingMode);
and

and APValue.cpp 69
V.convert(llvm::APFloat::IEEEdouble,
llvm::APFloat::rmNearestTiesToEven,
&ignored);

Are you building clang using the trunk llvm version ? That's the only supported way to build it currently (and it works with the current trunk: rev 60096).