Make the Preprocessor more memory efficient and improve macro instantiation diagnostics.

When a macro instantiation occurs, reserve a SLocEntry chunk with length the
full length of the macro definition source. Set the spelling location of this chunk
to point to the start of the macro definition and any tokens that are lexed directly
from the macro definition will get a location from this chunk with the appropriate offset.

For any tokens that come from argument expansion, '##' paste operator, etc. have their
instantiation location point at the appropriate place in the instantiated macro definition
(the argument identifier and the '##' token respectively).
This improves macro instantiation diagnostics:

Before:

t.c:5:9: error: invalid operands to binary expression ('struct S' and 'int')
int y = M(/);
        ^~~~
t.c:5:11: note: instantiated from:
int y = M(/);
          ^

After:

t.c:5:9: error: invalid operands to binary expression ('struct S' and 'int')
int y = M(/);
        ^~~~
t.c:3:20: note: instantiated from:
\#define M(op) (foo op 3);
                ~~~ ^  ~
t.c:5:11: note: instantiated from:
int y = M(/);
          ^

The memory savings for a candidate boost library that abuses the preprocessor are:

- 32% less SLocEntries (37M -> 25M)
- 30% reduction in PCH file size (900M -> 635M)
- 50% reduction in memory usage for the SLocEntry table (1.6G -> 800M)

llvm-svn: 134587