[clangd] Drop usage of PreambleStatCache in scanPreamble

Summary: It was used inside buildCompilerInvocation to speed up stats. But
preambleStatCache doesn't contain stat information performed while
building compiler invocation. So it was an unnecessary optimization.

Furthermore, buildCompilerInvocation in scanPreamble doesn't need to
find gcc installation, include paths and such, as it is only trying to
lex directives. Hence we are passing an empty FS to get rid of any
redundant IO.

Reviewers: sammccall

Subscribers: ilya-biryukov, javed.absar, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81719