ProfileData: Introduce InstrProfWriter using the naive text format

This isn't a format we'll want to write out in practice, but moving it
to the writer library simplifies llvm-profdata and isolates it from
further changes to the format.

This also allows us to update the tests to not rely on the text output
format.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204489 91177308-0d34-0410-b5e6-96231b3b80d8

lib/ProfileData/CMakeLists.txt

@@ -1,6 +1,7 @@
 add_llvm_library(LLVMProfileData
   InstrProf.cpp
   InstrProfReader.cpp
+  InstrProfWriter.cpp
 
   LINK_LIBS
   LLVMSupport