Cleanup version symbol macros and attributes/declspecs

1) Get rid of xaliasify, xexpand and xversionify for KMP_EXPAND_NAME and
KMP_VERSION_SYMBOL. KMP_VERSION_SYMBOL is a combination of xaliasify and
xversionify.

2) Put all attribute and __declspec definitions in kmp_os.h

Differential Revision: https://reviews.llvm.org/D39516

llvm-svn: 317636