[NFC][Clang] Fix potential dereferencing of nullptr (#85944)

This patch replaces getAs<> with castAs<> to resolve potential static
analyzer bugs for

1. Dereferencing a pointer issue with nullptr FPT when calling
ResolveExceptionSpec() in
    checkEscapingByref(clang::VarDecl *, clang::Sema &).

3. Dereferencing a pointer issue with nullptr ElementTy->getAs() when
calling getElementType() in
clang::Sema::SemaBuiltinFPClassification(clang::CallExpr *, unsigned
int).

4. Dereferencing a pointer issue with nullptr ConvType->getAs() when
calling getKeyword() in
clang::Sema::ActOnConversionDeclarator(clang::CXXConversionDecl *).