[CodeGen] Initialize large arrays by copying from a global

Currently, clang compiles explicit initializers for array elements
into series of store instructions. For large arrays of built-in types
this results in bloated output code and significant amount of time
spent on the instruction selection phase. This patch fixes the issue
by initializing such arrays with global constants that store the
binary image of the initializer.

Differential Revision: https://reviews.llvm.org/D43181