SourceManager: Vectorize ComputeLineNumbers for SSE2.

This method is very hot, it is called when emitting diagnostics, in -E mode
and for many #pragma handlers. It scans through the whole source file to
count newlines, records and caches them in a vector.

The speedup from vectorization isn't very large, as we fall back to bytewise
scanning when we hit a newline. There might be a way to avoid leaving the sse
loop but everything I tried didn't work out because a call to push_back
clobbers xmm registers.

About 2% speedup on average on "clang -E > /dev/null" of all .cpp files in
clang's lib/Sema.

llvm-svn: 154204