[PGO] PGOFuncName in LTO optimizations

PGOFuncNames are used as the key to retrieve the Function definition from the
MD5 stored in the profile. For internal linkage function, we prefix the source
file name to the PGOFuncNames. LTO's internalization privatizes many global linkage
symbols. This happens after value profile annotation, but those internal
linkage functions should not have a source prefix. To differentiate compiler
generated internal symbols from original ones, PGOFuncName meta data are
created and attached to the original internal symbols in the value profile
annotation step. If a symbol does not have the meta data, its original linkage
must be non-internal.

Also add a new map that maps PGOFuncName's MD5 value to the function definition.

Differential Revision: http://reviews.llvm.org/D17895

llvm-svn: 264902