PGO: Use the main file name to help distinguish functions with local linkage.

In addition, for all functions, use the name from the llvm::Function to
identify the function in the profile data. Compute that "function name",
including the file name for local functions, once when assigning the PGO
counters and store it in the CodeGenPGO class.

Move the code to add InlineHint and Cold attributes out of StartFunction(),
because the "function name" string isn't available at that point.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203075 91177308-0d34-0410-b5e6-96231b3b80d8