Add support for floating-point option `ffp-eval-method` and for

clang/docs/LanguageExtensions.rst

clang/docs/UsersManual.rst

clang/include/clang/Basic/DiagnosticLexKinds.td

clang/include/clang/Basic/DiagnosticParseKinds.td

clang/include/clang/Basic/FPOptions.def

clang/include/clang/Basic/LangOptions.def

clang/include/clang/Basic/LangOptions.h

clang/include/clang/Basic/TargetInfo.h

clang/include/clang/Driver/Options.td

clang/include/clang/Lex/Preprocessor.h

clang/include/clang/Parse/Parser.h

clang/include/clang/Sema/Sema.h

clang/lib/Basic/Targets/OSTargets.h

clang/lib/Basic/Targets/X86.h

clang/lib/Driver/ToolChains/Clang.cpp

clang/lib/Frontend/InitPreprocessor.cpp

clang/lib/Lex/PPMacroExpansion.cpp

clang/lib/Lex/Preprocessor.cpp

clang/lib/Parse/ParsePragma.cpp

clang/lib/Parse/ParseStmt.cpp