[clang-tidy] TransformerClangTidyCheck: change choice of location for diagnostic message.

Summary:
This patch changes the location specified to the
`ClangTidyCheck::diag()`. Currently, the beginning of the matched range is
used. This patch uses the beginning of the first fix's range.  This change both
simplifies the code and (hopefully) gives a more intuitive result: the reported
location aligns with the fix(es) provided, rather than the (arbitrary) range of
the rule's match.

N.B. this patch will break the line offset numbers in lit tests if the first fix
is not at the beginning of the match.

Reviewers: gribozavr

Subscribers: xazax.hun, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66676

llvm-svn: 369914