Teach -Wuninitialized to recognize __attribute__((analyzer_noreturn))

Teach -Wuninitialized to recognize __attribute__((analyzer_noreturn))
for halting the propagation of uninitialized value tracking along
a path.  Unlike __attribute__((noreturn)), this attribute (which
is used by clients of the static analyzer) can be used to annotate
functions that essentially never return, but in rare cares may be
allowed to return for (special) debugging purposes.  This attribute
has been shown in reducing false positives in the static analyzer
by pruning false postives, and is equally applicable here.

Handling this attribute in the CFG itself is another option, but
this is not something all clients (e.g., possibly -Wunreachable-code)
would want to see.

Addresses <rdar://problem/12281583>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@163681 91177308-0d34-0410-b5e6-96231b3b80d8