[ADT] Move away from __attribute__((always_inline)).

Some member functions of StringRef/SmallVector/StringSwitch
are marked with the `always_inline` attribute. The result
is that  the body of these functions is not emitted, hence the
debugger can't evaluate them (a typical example is
StringRef::size()), even if the code is built with `-O0`.

The main driver behind this was that of getting faster turnaround
when running `check-llvm`. A previous commit clarifies how to
get good performance when running the testsuite, so we can
get rid of the attribute here.

An alternative approach considered was that of using attribute `used`,
but in the end we preferred to not slap yet another attribute on
these functions.

llvm-svn: 351891