Fix #51194

This PR fixes a regression introduced in
#49294, so I believe it should be
backported to v1.10.
In the current code, completion of `qux(foo, bar.` is detected by
parsing `foo(qux, bar` as an incomplete expression, and then looking for
the sub-expression to complete (here, `bar.`). This approach fails
however for infix calls, since completing `foo + bar.` starts by parsing
`foo + bar`, which is a complete call expression, and so the code
behaves as if completing `(foo + bar).` instead of `bar.`. This leads to
the current problematic behaviour:
```julia
julia> Complex(1, 3) + (4//5).#TAB
im
re
```
which would be correct for `(Complex(1, 3) + (4//5)).#TAB`, but here we
expect
```julia
julia> Complex(1, 3) + (4//5).#TAB
den
num
```

This PR fixes that by trying to detect infix calls. In the long term,
all this ad-hoc and probably somewhat wrong string processing should be
replaced by proper use of `JuliaSyntax` (as mentioned in
#49294 (comment),
#50817 (comment)
and probably other places), but for now at least this fixes the
regression.

(cherry picked from commit e949236)

Backported PRs:
- [x] #50932 <!-- types: fix hash values of Vararg -->
- [x] #50975 <!-- Use rr-safe `nopl; rdtsc` sequence -->
- [x] #50989 <!-- fix incorrect results in `expm1(::Union{Float16,
Float32})` -->
- [x] #51284 <!-- Avoid infinite loop when doing SIGTRAP in arm64-apple
-->
- [x] #51332 <!-- Add s4 field to Xoshiro -->
- [x] #51397 <!-- call Pkg precompile hook in latest world -->
- [x] #51405 <!-- Remove fallback that assigns a module to inlined
frames. -->
- [x] #51491 <!-- Throw clearer ArgumentError for strip with two string
args -->
- [x] #51531 <!-- fix `_tryonce_download_from_cache` (busybox.exe
download error) -->
- [x] #51541 <!-- Fix string index error in tab completion code -->
- [x] #51530 <!-- Don't mark nonlocal symbols as hidden -->
- [x] #51557 <!-- Fix last startup & shutdown precompiles -->
- [x] #51512 <!-- avoid limiting Type{Any} to Type -->
- [x] #51595 <!-- reset `maxprobe` on `empty!` -->
- [x] #51582 <!-- Aggressive constprop in LinearAlgebra.wrap -->
- [x] #51592 <!-- correctly track element pointer in heap snapshot -->
- [x] #51326 <!-- complete false & true more generally as vals -->
- [x] #51376 <!-- make `hash(::Xoshiro)` compatible with `==` -->
- [x] #51557 <!-- Fix last startup & shutdown precompiles -->
- [x] #51845 
- [x] #51840 
- [x] #50663 <!-- Fix Expr(:loopinfo) codegen -->
- [x] #51863 <!-- LLVM 15.0.7-9 -->

Contains multiple commits, manual intervention needed:

- [ ] #51035 <!-- refactor GC scanning code to reflect jl_binding_t are
now first class -->
- [ ] #51092 <!-- inference: fix bad effects for recursion -->

Non-merged PRs with backport label:
- [ ] #51479 <!-- prevent code loading from lookin in the versioned
environment when building Julia -->
- [ ] #51414 <!-- improvements on GC scheduler shutdown -->
- [ ] #51366 <!-- Handle infix operators in REPL completion -->
- [ ] #50919 <!-- Code loading: do the "skipping mtime check for stdlib"
check regardless of the value of `ispath(f)` -->
- [ ] #50824 <!-- Add some aliasing warnings to docstrings for mutating
functions in Base -->
- [ ] #49805 <!-- Limit TimeType subtraction to AbstractDateTime -->