PGO: Add function to reset counters at runtime

Adding __llvm_pgo_reset_counters(), which sets all the counters to 0.

<rdar://problem/15943240>

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@204386 91177308-0d34-0410-b5e6-96231b3b80d8
Add vim swap files to .gitignore

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@204387 91177308-0d34-0410-b5e6-96231b3b80d8