docs: Fix up HTML links to proper reST links.

clang/docs/InternalsManual.rst

clang/docs/JSONCompilationDatabase.rst

clang/docs/LanguageExtensions.rst

clang/docs/LibASTMatchers.rst

clang/docs/LibTooling.rst

clang/docs/ObjectiveCLiterals.rst

clang/docs/PCHInternals.rst

clang/docs/Tooling.rst