[SourceManager] Improve getFileIDLoaded.

Similar to getFileIDLocal patch, but for the version for load module.

Test with clangd (building AST with preamble), FileID scans in binary
search is reduced:

SemaExpr.cpp: 142K -> 137K (-3%)
FindTarget.cpp: 368K -> 343K (-6%)

Differential Revision: https://reviews.llvm.org/D135258