[clangd] Fix clangd crash when including a header

Fixes https://github.com/clangd/clangd/issues/819

SourceLocation of macros change when a header file is included above it. This is not checked when creating a PreamblePatch, resulting in reusing previously built preamble with an incorrect source location for the macro in the example test case.
This patch stores the SourceLocation in the struct TextualPPDirective so that it gets checked when comparing old vs new preambles.

Also creates a preamble patch for code completion parsing so that clangd does not crash when following the example test case with a large file.

Reviewed By: kadircet

Differential Revision: https://reviews.llvm.org/D108045