Add GetAddrOfConstantCString method

 - Returns addr of constant for argument + '\0'.
 - I couldn't think of a better name.
 - Move appropriate users of GetAddrOfConstantString to this.

Rename getStringForStringLiteral to GetStringForStringLiteral.

Add GetAddrOfConstantStringFromLiteral
 - This combines GetAddrOfConstantString and
   GetStringForStringLiteral. This method can be, but is not yet, more
   efficient.

Change GetAddrOfConstantString to not add terminating '\0'
 - <rdar://problem/6140956>

llvm-svn: 54768