Our {Vector} template provides both {start} and {begin} methods. They
return exactly the same value. Since the {begin} method is needed for
iteration, and is also what standard containers provide, this CL
switches all uses of the {start} method to use {begin} instead.

Patchset 1 was auto-generated by using this clang AST matcher:
    callExpr(
        callee(
          cxxMethodDecl(
            hasName("start"),
            ofClass(hasName("v8::internal::Vector")))
        ),
        argumentCountIs(0))

Patchset 2 was created by running clang-format. Patchset 3 then
removes the now unused {Vector::start} method.

R=jkummerow@chromium.org
TBR=mstarzinger@chromium.org,yangguo@chromium.org,verwaest@chromium.org

Bug: v8:9183
Change-Id: Id9f01c92870872556e2bb3f6d5667463b0e3e5c6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1587381Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61081}