Refactor profile summary support code. NFC.

Summary computation is not just for instrumented profiling and so I have moved
the ProfileSummary class to ProfileCommon.h (named so to allow code unrelated
to summary but common to instrumented and sampled profiling to be placed there)

Differential Revision: http://reviews.llvm.org/D16661

llvm-svn: 259846
6 files changed