[Support] Add type-safe alternative to llvm::format()

llvm::format() is somewhat unsafe. The compiler does not check that integer
parameter size matches the %x or %d size and it does not complain when a 
StringRef is passed for a %s.  And correctly using a StringRef with format() is  
ugly because you have to convert it to a std::string then call c_str().
 
The cases where llvm::format() is useful is controlling how numbers and
strings are printed, especially when you want fixed width output.  This
patch adds some new formatting functions to raw_streams to format numbers
and StringRefs in a type safe manner. Some examples:

   OS << format_hex(255, 6)        => "0x00ff"
   OS << format_hex(255, 4)        => "0xff"
   OS << format_decimal(0, 5)      => "    0"
   OS << format_decimal(255, 5)    => "  255"
   OS << right_justify(Str, 5)     => "  foo"
   OS << left_justify(Str, 5)      => "foo  "

llvm-svn: 218463

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