SamplePGO - Reduce memory utilization by 10x.

DenseMap is the wrong data structure to use for sample records and call
sites.  The keys are too large, causing massive core memory growth when
reading profiles.

Before this patch, a 21Mb input profile was causing the compiler to grow
to 3Gb in memory.  By switching to std::map, the compiler now grows to
300Mb in memory.

There still are some opportunities for memory footprint reduction. I'll
be looking at those next.

llvm-svn: 255389