For a description of the extension, see the GNAT reference manual.
An effort has been made to minimize the impact on parsing error recovery.
Basic benchmarking was performed and showed no observable performance
impact.
Some tests mean to trigger a parsing error in Ada sources. For them,
disable the printing of parsing errors in the inline playground to avoid
polluting baselines.
This commit turns on the "PLE root" Langkit mechanism to be able to run
PLE on each compilation unit independently, even if compilation units
are stored in the same source file.
In addition, it extends all the unit provider where this is possible
(Auto_Provider, Project_Provider for GPR2) so that resolving a unit
yields a filename *plus* the compilation unit index in that source file.
Finally, it adds several testcases to exercize these features.
Move most of "ada/*" to the root directory (this makes sense, as this
repository has been dedicated to Libadalang for years), and rename
"ada/language" to "ada".
TN: T914-010
