[mlir-lsp-server] Only use one MLIRContext per MLIRTextFile

A text file may be comprised of many different "chunks", when
the input file contains the `// -----` split markers. We don't
need to use a unique MLIRContext per chunk, as having
separate contexts is intended to allow for easy unloading of
unused data and all chunks have the same lifetime (tied to the
input file). This commit uses one context for the entire file,
greatly reducing memory consumption in certain situations (up
to 70%).

Differential Revision: https://reviews.llvm.org/D107488