[flang] Add FoldingContext to SemanticsContext

FoldingContext is now part of SemanticsContext. It is created at the
beginning with a default-constructed CharBlock as the location in its
ContextualMessages.

Add PushLocation() to ContextualMessages to remember the previous source
location and set a new one. The old one is restored when the returned
object goes out of scope.

SemanticsContext is now the only state passed in to class ExprAnalyzer,
class Mutator, AnalyzeExpr(), AnalyzeExpressions().

Add Say() convenience functions for reporting errors to ExprAnalyzer.

Original-commit: flang-compiler/f18@70c499ffc4a1f09a680d636f59938f42ec451ef3
Reviewed-on: https://github.com/flang-compiler/f18/pull/215
Tree-same-pre-rewrite: false