Add function entry count metadata.

Summary:
This adds three Function methods to handle function entry counts:
setEntryCount() and getEntryCount().

Entry counts are stored under the MD_prof metadata node with the name
"function_entry_count". They are unsigned 64 bit values set by profilers
(instrumentation and sample profiler changes coming up).

Added documentation for new profile metadata and tests.

Reviewers: dexonsmith, bogner

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9628

llvm-svn: 237260