[lit] Support %if ... %else syntax for RUN lines

This syntax allows to modify RUN lines based on features
available. For example:

    RUN: ... | FileCheck %s --check-prefix=%if windows %{CHECK-W%} %else %{CHECK-NON-W%}
    CHECK-W: ...
    CHECK-NON-W: ...

The whole command can be put under %if ... %else:

    RUN: %if tool_available %{ %tool %} %else %{ true %}

or:

    RUN: %if tool_available %{ %tool %}

If tool_available feature is missing, we'll have an empty command in
this RUN line.  LIT used to emit an error for empty commands, but now
it treats such commands as nop in all cases.

Multi-line expressions are also supported:

    RUN: %if tool_available %{ \
    RUN:   %tool               \
    RUN: %} %else %{           \
    RUN:   true                \
    RUN: %}

Background and motivation:
D121727 [NVPTX] Integrate ptxas to LIT tests
https://reviews.llvm.org/D121727

Differential Revision: https://reviews.llvm.org/D122569