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This PR implements a CC1 flag `-dump-minimization-hints`.
The flag allows to specify a file path to dump ranges of deserialized
declarations in `ASTReader`. Example usage:
```
clang -Xclang=-dump-minimization-hints=/tmp/decls -c file.cc -o file.o
```
Example output:
```
// /tmp/decls
{
"required_ranges": [
{
"file": "foo.h",
"range": [
{
"from": {
"line": 26,
"column": 1
},
"to": {
"line": 27,
"column": 77
}
}
]
},
{
"file": "bar.h",
"range": [
{
"from": {
"line": 30,
"column": 1
},
"to": {
"line": 35,
"column": 1
}
},
{
"from": {
"line": 92,
"column": 1
},
"to": {
"line": 95,
"column": 1
}
}
]
}
]
}
```
Specifying the flag creates an instance of
`DeserializedDeclsSourceRangePrinter`, which dumps ranges of deserialized
declarations to aid debugging and bug minimization (we use is as input to [C-Vise](https://github.com/emaxx-google/cvise/tree/multifile-hints).
Required ranges are computed from source ranges of Decls.
`TranslationUnitDecl`, `LinkageSpecDecl` and `NamespaceDecl` are ignored
for the sake of this PR.
Technical details:
* `DeserializedDeclsSourceRangePrinter` implements `ASTConsumer` and
`ASTDeserializationListener`, so that an object of
`DeserializedDeclsSourceRangePrinter` registers as its own listener.
* `ASTDeserializationListener` interface provides the `DeclRead`
callback that we use to collect the deserialized Decls.
Printing or otherwise processing them as this point is dangerous, since
that could trigger additional deserialization and crash compilation.
* The collected Decls are processed in `HandleTranslationUnit` method of
`ASTConsumer`. This is a safe point, since we know that by this point
all the Decls needed by the compiler frontend have been deserialized.
* In case our processing causes further deserialization, `DeclRead` from
the listener might be called again. However, at that point we don't
accept any more Decls for processing.