Reduce stack usage of TreeTransform.

Without this patch, TreeTransform::TransformExpr uses a ridiculous amount of
stack space (around 5000 bytes).  Preventing inlining brings the stack usage
down to something sane.

On a testcase I have, on my computer, this allows changing -ftemplate-depth
from 210 to around 750 before we crash.  I'm not sure I should commit the
testcase, though: I don't want to cause test failures on platforms with less
stack space available.

<rdar://problem/14098189>.