[clangd] locateMacroAt handles patched macros

[clangd] locateMacroAt handles patched macros

Summary: Depends on D79992.

This patch changes locateMacroAt to perform #line directive substitution
for macro identifier locations.

We first check whether a location is inside a file included through
built-in header. If so we check whether line directive maps it back to
the main file, and afterwards use TokenBuffers to find exact location of
the identifier on the line.

Instead of performing the mapping in locateMacroAt, we could also store
a mapping inside the ParsedAST whenever we use a patched preamble. But
that would imply adding more responsibility to ParsedAST and paying for
the mapping even when it is not going to be used.

====

Go-To-Definition:

Later on these locations are used for serving go-to-definition requests,
this enables jumping to definition inside the preamble section in
presence of patched macros.

=====

Go-To-Refs:

Macro references in main file are collected separetely and stored as a
map from macro's symbol id to reference ranges. Those ranges are
computed inside PPCallbacks, hence we don't have access to TokenBuffer.

In presence of preamble patch, any reference to a macro inside the
preamble section will unfortunately have the wrong range. They'll point
into the patch rather than the main file. Hence during findReferences,
we won't get any ranges reported for those.

Fixing those requires:
- Lexing the preamble section to figure out "real range" of a patched
  macro definition
- Postponing range/location calculations until a later step in which we
  have access to tokenbuffers.

This patch trades some accuracy in favor of code complexity. We don't do
any patching for references inside the preamble patch but get any
reference inside the main file for free.

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80198