[ProfileData] Use a different data structure to save memory.

This change swaps FunctionSamples to a std::map. This saves us around
17% of the memory required to parse sample profiles. To put hard numbers
on this, clang now eats around 1.3GB of RAM instead of 1.6GB while
parsing a 50MB profile.

The CPU time taken by a large profile merge (3.1GB of data across 226
files) is also reduced by ~11% by this patch (1:09.08 vs 1:01.11).

This was split out at the request of reviewers in D41152.

llvm-svn: 320764