[clang-query] Load queries and matchers from file during REPL cycle (#90603)

The clang-query tool has the ability to execute or pre-load queries from
a file when the tool is launched, but doesn't have the ability to do the
same from the interactive REPL prompt. Because the prompt also doesn't
seem to allow multi-line matchers, this can make prototyping and
iterating on more complicated matchers difficult.

Supporting a dynamic load at REPL time allows the cost of reading the
compilation database and building the AST to be imposed just once, and
allows faster prototyping.