Refactor StaticAnalyzer to use `clang::SarifDocumentWriter`

Refactor StaticAnalyzer to use clang::SarifDocumentWriter for
serializing sarif diagnostics.

Uses clang::SarifDocumentWriter to generate SARIF output in the
StaticAnalyzer.

Various bugfixes are also made to clang::SarifDocumentWriter.

Summary of changes:

clang/lib/Basic/Sarif.cpp:
  * Fix bug in adjustColumnPos introduced from prev move, it now uses
    FullSourceLoc::getDecomposedExpansionLoc which provides the correct
    location (in the presence of macros) instead of
    FullSourceLoc::getDecomposedLoc.
  * Fix createTextRegion so that it handles caret ranges correctly,
    this should bring it to parity with the previous implementation.

clang/test/Analysis/diagnostics/Inputs/expected-sarif:
  * Update the schema URL to the offical website
  * Add the emitted defaultConfiguration sections to all rules
  * Annotate results with the "level" property

clang/lib/StaticAnalyzer/Core/SarifDiagnostics.cpp:
  * Update SarifDiagnostics class to hold a clang::SarifDocumentWriter
    that it uses to convert diagnostics to SARIF.