Refactor how attributes are set on values.

 - Pull out SetCommonAttributes, which handles the things common to
   aliases, methods, functions, and variables.

 - Pull out SetLLVMFunctionAttributesForDefinition, which handles the
   LLVM attributes which we only want to apply to a definition (like
   noinline and alwaysinline).

 - Kill SetGVDeclarationAttributes (inlined into SetFunctionAttributes
   and specialized).

 - Kill SetFunctionAttributesForDefinition (inlined into sole caller).

 - Inline SetGVDefinitionAttributes into SetMethodAttributes and
   specialize.

 - Rename SetGVDefinitionAttributes to SetFunctionDefinitionAttributes.

This is supposed to be a no functionality change commit, but I may
have made a mistake.

llvm-svn: 69036