Replaced ASTContext::maxFloatingType() with ASTContext::compareFloatingType().

Changed Sema::UsualArithmeticConversions to use the new API.

This fixes the following case...

_Complex double X;
double y;

void foo() {
  X = X + y;
}

[dylan:~/llvm/tools/clang] admin% ../../Debug/bin/clang complex.c -parse-ast-dump
Read top-level variable decl: 'X'
Read top-level variable decl: 'y'

void foo()
(CompoundStmt 0x2605cc0
  (BinaryOperator 0x2605ca0 '_Complex double' '='
    (DeclRefExpr 0x2605c10 '_Complex double' Decl='X' 0x2605ab0)
    (BinaryOperator 0x2605c80 '_Complex double' '+'
      (DeclRefExpr 0x2605c30 '_Complex double' Decl='X' 0x2605ab0)
      (ImplicitCastExpr 0x2605c70 '_Complex double'
        (DeclRefExpr 0x2605c50 'double' Decl='y' 0x2605ae0)))))

llvm-svn: 41483