This patch uses data() and size() within StringRef instead of Data and
Length.  This makes it easier to replace Data and Length with
std::string_view in the future, which in turn allows us to forward
most of StringRef functions to the counterparts in std::string_view.

S.substr(N) is simpler than S.slice(N, StringRef::npos) and
S.slice(N, S.size()). Also, substr is probably better recognizable
than slice thanks to std::string_view::substr.

startswith/endswith wrap starts_with/ends_with and will eventually go away (to more closely match string_view)

This will be required to allow arbitrary precision support to
FileCheck's numeric variables and expressions. Note: as per
getAsInteger(), this does not support negative value. If there is
interest for that it can be added in a separate patch.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D150878

This patch migrates uses of StringRef::{starts,ends}with_insensitive
to StringRef::{starts,ends}_with_insensitive so that we can use names
similar to those used in std::string_view.  I'm planning to deprecate
StringRef::{starts,ends}with_insensitive once the migration is
complete across the code base.

Differential Revision: https://reviews.llvm.org/D150426

These are identical in terms of functionality and performance (checked
libc++ and libstdc++). We could do the same for rfind, but that actually
has a off-by one on its position argument.

StringRef::find(StringRef) seems to be quite a bit more optimized than
the standard library one, so leave it alone.