[analyzer] Syntactic matcher for leaks associated with run loop and autoreleasepool

A checker for detecting leaks resulting from allocating temporary
autoreleasing objects before starting the main run loop.

Checks for two antipatterns:

1. ObjCMessageExpr followed by [[NARunLoop mainRunLoop] run] in the same
autorelease pool.

2. ObjCMessageExpr followed by [[NARunLoop mainRunLoop] run] in no
autorelease pool.

Happens-before relationship is modeled purely syntactically.

rdar://39299145

Differential Revision: https://reviews.llvm.org/D49528

llvm-svn: 337876