[clang-tools-extra] [clangd] Harden incomingCalls() against possible misinterpretation of a range as pertaining to the wrong file (PR #111616)

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

Updated patch with the following changes:

 * Resolve the potential UAF by storing `clangd::Location` rather than `SymbolLocation` in the `CallsIn` map
 * Add a testcase for the scenario from [this thread](https://github.com/llvm/llvm-project/pull/111616#discussion_r1810411700)

Should be ready for another round of review.

https://github.com/llvm/llvm-project/pull/111616