[Sema] Make helper in TreeTransform.h 'inline' instead of 'static'. NFC

Summary:
There seems to be no evidence that having internal linkage for the function
was intentional. Since 'static' functions are normally used only in .cpp
files, using 'inline' in the header file is more appropriate.

Reviewers: Anastasia

Reviewed By: Anastasia

Subscribers: merge_guards_bot, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69242