[leaks] Fix a leak of a basic block when we successfully fold a switch
condition to a constant and emit only the relevant statement. In that
case, we were previously creating the epilog jump destination, a cleanup
scope, and emitting any condition variable into it. Instead, we can emit
the condition variable (if we have one) into the cleanup scope used for
the entire folded case sequence. We avoid creating a jump dest, a basic
block, and an extra cleanup scope. Win!

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@207888 91177308-0d34-0410-b5e6-96231b3b80d8
MS ABI x64: Pass small objects with dtors but no copy ctors directly

Passing objects directly (in registers or memory) creates a second copy
of the object in the callee.  The callee always destroys its copy, but
we also have to destroy any temporary created in the caller.  In other
words, copy elision of these kinds of objects is impossible.

Objects larger than 8 bytes with non-trivial dtors and trivial copy
ctors are still passed indirectly, and we can still elide copies of
them.

Fixes PR19640.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@207889 91177308-0d34-0410-b5e6-96231b3b80d8
Rewrite NRVO determination. Track NRVO candidates on the parser Scope and apply the NRVO candidate flag to all possible NRVO candidates here, and remove the flags in computeNRVO or upon template instantiation. A variable now has NRVO applied if and only if every return statement in that scope returns that variable. This is nearly optimal.

Performs NRVO roughly 7% more often in a bootstrap build of clang. Patch co-authored by Richard Smith.


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@207890 91177308-0d34-0410-b5e6-96231b3b80d8