[compiler-rt] Introduce the notion of an interceptor trampoline

[compiler-rt] Introduce the notion of an interceptor trampoline

To make the interceptor implementation more flexible, allowing for 2
levels of indirection instead of just 1 in the current scheme (where the
intercepted function aliases the interceptor implementation), introduce
the notion of an interceptor "trampoline".

A trampoline may be a real function (and not just an alias, where
aliases of aliases do not work), which will simply forward to the
interceptor implementation; the intercepted function will then alias the
trampoline:

	func -[alias]-> trampoline -[call]-> interceptor

Make the necessary changes to prepare for introducing real trampolines.

This change does not yet introduce any real trampolines, and so
trampoline == interceptor, and we currently still just have:

	func -[alias]-> interceptor

NFC.

Reviewed By: dvyukov, vitalybuka, MaskRay

Differential Revision: https://reviews.llvm.org/D151316