Fix most MSVC warnings in compiler-rt profiling library

Here's the warnings and how they were fixed:

- InstrProfilingUtil.c(110): warning C4013: '_open_osfhandle' undefined; assuming extern returning int

Include io.h to get the prototype.

- warning C4005: 'FILE_MAP_EXECUTE': macro redefinition

Stop trying to support pre-XP versions of Windows, don't attempt to
define this macro.

- InstrProfilingWriter.c(271): warning C4221: nonstandard extension used: 'Data': cannot be initialized using address of automatic variable 'Header'
- InstrProfilingWriter.c(275): warning C4221: nonstandard extension used: 'Data': cannot be initialized using address of automatic variable 'Zeroes'

Turn this warning off. This is definitely legal in C++, all compilers
accept it, and I only have room for half of one language standard in my
brain.

- InstrProfilingValue.c(320): warning C4113: 'uint32_t (__cdecl *)()' differs in parameter lists from 'uint32_t (__cdecl *)(void)'

Fix this with an explicit (void) in the prototype.

- InstrProfilingMerge.c.obj : warning LNK4006: _VPMergeHook already defined in InstrProfilingMergeFile.c.obj; second definition ignored

Last remaining warning. This is from linking a selectany definition with
a strong definition. We need to sort out weak symbols in compiler-rt in
general, though.

llvm-svn: 273026