struct Location size

[Walter Bright]

This PR https://github.com/dlang/dmd/pull/15199 reduces its size by 8 bytes, 
resulting in about 20Mb of memory savings compiling druntime, according to @dkorpel.

It is currently:

struct Loc
{
   uint linnum; // line number, starting from 1
   ushort charnum; // utf8 code unit index relative to start of line, starting 
from 1
   ushort fileIndex; // index into filenames[], starting from 1 (0 means no 
filename)
}

which is 8 bytes.

If it was 4 bytes, it could still store 4 billion unique locations, which ought 
to be enough for everyone. I toyed around with various encodings:

  6 bits for column - 1..64
15 bits for line - 1..32768
11 bits for file - 2047

I also thought of schemes like having the sign bit set an alternate encoding scheme.

So, for great glory, can anyone come up with a clever scheme that uses only 32 bits?