Replace some typically large vectors with SmallVector.

This may seem counter-intuitive but the POD-like optimization helps when the
vectors grow into multimegabyte buffers. SmallVector calls realloc which knows
how to twiddle virtual memory bits and avoids large copies.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175906 91177308-0d34-0410-b5e6-96231b3b80d8
[preprocessing record] Have the MacroDefinitions map point to the MacroDefinition object instead
its index in the preprocessed entities vector.

This is because the order of the entities in the vector can change in some (uncommon) cases.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175907 91177308-0d34-0410-b5e6-96231b3b80d8