[CodeGen] Emit a helper function for __builtin_os_log_format to reduce

code size.

Currently clang expands a call to __builtin_os_log_format into a long
sequence of instructions at the call site, causing code size to
increase in some cases.

This commit attempts to reduce code size by emitting a helper function
that can be shared by calls to __builtin_os_log_format with similar
formats and arguments. The helper function has linkonce_odr linkage to
enable the linker to merge identical functions across translation units.
Attribute 'noinline' is attached to the helper function at -Oz so that
the inliner doesn't inline functions that can potentially be merged.

This commit also fixes a bug where the generated IR writes past the end
of the buffer when "%m" is the last specifier appearing in the format
string passed to __builtin_os_log_format.

Original patch by Duncan Exon Smith.

rdar://problem/34065973
rdar://problem/34196543

Differential Revision: https://reviews.llvm.org/D38606

llvm-svn: 315045