PGO: Add explicit static initialization
Instead of relying on explicit static initialization from translation units, create a new file, InstrProfilingRuntime.cc, with an __llvm_pgo_runtime variable. After this commit (and its pair in clang), the driver will create a use of this variable. Unless the user defines their own version, the new object file will get pulled in, including that C++ static initialization that calls __llvm_pgo_register_write_atexit. The result is that, at least on Darwin, static initialization typically consists of a single function call, which registers a writeout functino atexit. Furthermore, users can skip even this behaviour by defining their own __llvm_pgo_runtime. <rdar://problem/15943240> llvm-svn: 204380
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